<?xml version="1.0" encoding="UTF-8"?>
<!-- Documents rendered in XBRL format prepared by Certent CDM XBRL Version: 10.3.0.0.26 (5021) using Fujitsu Interstage XWand -->
<!-- Validation status: Passed -->
<linkbase xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/instance http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd" xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xbrldt="http://xbrl.org/2005/xbrldt">
    <roleRef roleURI="http://www.ies-co.com/role/BusinessCombinationsAssetsDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_BusinessCombinationsAssetsDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/BusinessCombinationsDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_BusinessCombinationsDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/BusinessCombinationsProFormasDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_BusinessCombinationsProFormasDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/BusinessDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_BusinessDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/CommitmentsAndContingenciesDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_CommitmentsAndContingenciesDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/ConsolidatedStatementOfStockholdersEquity" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_ConsolidatedStatementOfStockholdersEquity" />
    <roleRef roleURI="http://www.ies-co.com/role/ControllingShareholderDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_ControllingShareholderDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/DebtDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_DebtDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/FairValueMeasurementsDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_FairValueMeasurementsDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/FairValueMeasurementsUnobservableInputsDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_FairValueMeasurementsUnobservableInputsDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/GoodwilAndIntangibleAssetsOtherIntangibleAssetsDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_GoodwilAndIntangibleAssetsOtherIntangibleAssetsDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/GoodwillAndIntangibleAssetsGoodwillRollForwardDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_GoodwillAndIntangibleAssetsGoodwillRollForwardDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/OperatingSegmentsDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_OperatingSegmentsDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/RevenueRecognitionDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_RevenueRecognitionDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/StockholdersEquityDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_StockholdersEquityDetails" />
    <roleRef roleURI="http://www.ies-co.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="iesc-20181231.xsd#ELRID_SubsequentEventsDetails" />
    <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" />
    <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" />
    <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" />
    <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" />
    <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" />
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/OperatingSegmentsDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_429" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CorporateMember" xlink:label="Locator_us-gaap_CorporateMember_430" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_429" xlink:to="Locator_us-gaap_CorporateMember_430" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_421" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommercialIndustrialMember" xlink:label="Locator_iesc_CommercialIndustrialMember_422" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_421" xlink:to="Locator_iesc_CommercialIndustrialMember_422" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_423" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommunicationsMember" xlink:label="Locator_iesc_CommunicationsMember_424" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_423" xlink:to="Locator_iesc_CommunicationsMember_424" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_425" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_InfrastructureSolutionsMember" xlink:label="Locator_iesc_InfrastructureSolutionsMember_426" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_425" xlink:to="Locator_iesc_InfrastructureSolutionsMember_426" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_427" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_ResidentialMember" xlink:label="Locator_iesc_ResidentialMember_428" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_427" xlink:to="Locator_iesc_ResidentialMember_428" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_419" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_420" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_StatementBusinessSegmentsAxis_419" xlink:to="Locator_us-gaap_SegmentDomain_420" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_451" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_452" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_StatementBusinessSegmentsAxis_451" xlink:to="Locator_us-gaap_SegmentDomain_452" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_413" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="Locator_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_414" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_413" xlink:to="Locator_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_414" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="Locator_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_417" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_418" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_417" xlink:to="Locator_us-gaap_StatementBusinessSegmentsAxis_418" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_431" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Revenues" xlink:label="Locator_us-gaap_Revenues_432" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_431" xlink:to="Locator_us-gaap_Revenues_432" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_433" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CostOfGoodsAndServicesSold" xlink:label="Locator_us-gaap_CostOfGoodsAndServicesSold_434" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_433" xlink:to="Locator_us-gaap_CostOfGoodsAndServicesSold_434" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_435" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GrossProfit" xlink:label="Locator_us-gaap_GrossProfit_436" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_435" xlink:to="Locator_us-gaap_GrossProfit_436" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_437" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpense" xlink:label="Locator_us-gaap_SellingGeneralAndAdministrativeExpense_438" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_437" xlink:to="Locator_us-gaap_SellingGeneralAndAdministrativeExpense_438" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_439" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationIntegrationRelatedCosts" xlink:label="Locator_us-gaap_BusinessCombinationIntegrationRelatedCosts_440" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_439" xlink:to="Locator_us-gaap_BusinessCombinationIntegrationRelatedCosts_440" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_441" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GainLossOnDispositionOfAssets" xlink:label="Locator_us-gaap_GainLossOnDispositionOfAssets_442" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_441" xlink:to="Locator_us-gaap_GainLossOnDispositionOfAssets_442" order="6.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_443" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingIncomeLoss" xlink:label="Locator_us-gaap_OperatingIncomeLoss_444" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_443" xlink:to="Locator_us-gaap_OperatingIncomeLoss_444" order="7.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_445" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Depreciation" xlink:label="Locator_us-gaap_Depreciation_446" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_445" xlink:to="Locator_us-gaap_Depreciation_446" order="8.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_447" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PaymentsToAcquireProductiveAssets" xlink:label="Locator_us-gaap_PaymentsToAcquireProductiveAssets_448" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_447" xlink:to="Locator_us-gaap_PaymentsToAcquireProductiveAssets_448" order="9.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="Locator_us-gaap_SegmentReportingInformationLineItems_449" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Assets" xlink:label="Locator_us-gaap_Assets_450" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentReportingInformationLineItems_449" xlink:to="Locator_us-gaap_Assets_450" order="10.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/DebtDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="Locator_us-gaap_LineOfCreditFacilityLineItems_485" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="Locator_us-gaap_LineOfCreditFacilityTable_486" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_LineOfCreditFacilityLineItems_485" xlink:to="Locator_us-gaap_LineOfCreditFacilityTable_486" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="Locator_us-gaap_LineOfCreditFacilityTable_487" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="Locator_us-gaap_CreditFacilityAxis_488" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_LineOfCreditFacilityTable_487" xlink:to="Locator_us-gaap_CreditFacilityAxis_488" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="Locator_us-gaap_CreditFacilityAxis_489" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="Locator_us-gaap_CreditFacilityDomain_490" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_CreditFacilityAxis_489" xlink:to="Locator_us-gaap_CreditFacilityDomain_490" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="Locator_us-gaap_CreditFacilityAxis_501" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="Locator_us-gaap_CreditFacilityDomain_502" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_CreditFacilityAxis_501" xlink:to="Locator_us-gaap_CreditFacilityDomain_502" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="Locator_us-gaap_CreditFacilityDomain_493" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="Locator_srt_MinimumMember_494" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_CreditFacilityDomain_493" xlink:to="Locator_srt_MinimumMember_494" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="Locator_us-gaap_CreditFacilityDomain_491" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="Locator_us-gaap_RevolvingCreditFacilityMember_492" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_CreditFacilityDomain_491" xlink:to="Locator_us-gaap_RevolvingCreditFacilityMember_492" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="Locator_us-gaap_LineOfCreditFacilityLineItems_495" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd" xlink:label="Locator_us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd_496" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LineOfCreditFacilityLineItems_495" xlink:to="Locator_us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd_496" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="Locator_us-gaap_LineOfCreditFacilityLineItems_497" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount" xlink:label="Locator_us-gaap_LettersOfCreditOutstandingAmount_498" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LineOfCreditFacilityLineItems_497" xlink:to="Locator_us-gaap_LettersOfCreditOutstandingAmount_498" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="Locator_us-gaap_LineOfCreditFacilityLineItems_499" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity" xlink:label="Locator_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_500" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LineOfCreditFacilityLineItems_499" xlink:to="Locator_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_500" order="3.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/CommitmentsAndContingenciesDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_571" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesTable" xlink:label="Locator_us-gaap_LossContingenciesTable_572" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_LossContingenciesLineItems_571" xlink:to="Locator_us-gaap_LossContingenciesTable_572" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesTable" xlink:label="Locator_us-gaap_LossContingenciesTable_575" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_LitigationCaseAxis" xlink:label="Locator_srt_LitigationCaseAxis_576" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_LossContingenciesTable_575" xlink:to="Locator_srt_LitigationCaseAxis_576" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_LitigationCaseAxis" xlink:label="Locator_srt_LitigationCaseAxis_585" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_LitigationCaseTypeDomain" xlink:label="Locator_srt_LitigationCaseTypeDomain_586" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_srt_LitigationCaseAxis_585" xlink:to="Locator_srt_LitigationCaseTypeDomain_586" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_LitigationCaseAxis" xlink:label="Locator_srt_LitigationCaseAxis_619" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_LitigationCaseTypeDomain" xlink:label="Locator_srt_LitigationCaseTypeDomain_620" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_srt_LitigationCaseAxis_619" xlink:to="Locator_srt_LitigationCaseTypeDomain_620" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_LitigationCaseTypeDomain" xlink:label="Locator_srt_LitigationCaseTypeDomain_587" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CapstoneMember" xlink:label="Locator_iesc_CapstoneMember_588" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_srt_LitigationCaseTypeDomain_587" xlink:to="Locator_iesc_CapstoneMember_588" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_LitigationCaseTypeDomain" xlink:label="Locator_srt_LitigationCaseTypeDomain_589" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_UsamriidmemberMember" xlink:label="Locator_iesc_UsamriidmemberMember_590" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_srt_LitigationCaseTypeDomain_589" xlink:to="Locator_iesc_UsamriidmemberMember_590" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesTable" xlink:label="Locator_us-gaap_LossContingenciesTable_577" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="Locator_us-gaap_StatementScenarioAxis_578" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_LossContingenciesTable_577" xlink:to="Locator_us-gaap_StatementScenarioAxis_578" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="Locator_us-gaap_StatementScenarioAxis_591" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="Locator_us-gaap_ScenarioUnspecifiedDomain_592" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_StatementScenarioAxis_591" xlink:to="Locator_us-gaap_ScenarioUnspecifiedDomain_592" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="Locator_us-gaap_StatementScenarioAxis_617" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="Locator_us-gaap_ScenarioUnspecifiedDomain_618" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_StatementScenarioAxis_617" xlink:to="Locator_us-gaap_ScenarioUnspecifiedDomain_618" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="Locator_us-gaap_ScenarioUnspecifiedDomain_593" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScenarioForecastMember" xlink:label="Locator_us-gaap_ScenarioForecastMember_594" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ScenarioUnspecifiedDomain_593" xlink:to="Locator_us-gaap_ScenarioForecastMember_594" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_597" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingencyNameOfPlaintiff" xlink:label="Locator_us-gaap_LossContingencyNameOfPlaintiff_598" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LossContingenciesLineItems_597" xlink:to="Locator_us-gaap_LossContingencyNameOfPlaintiff_598" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_599" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingencyDamagesSoughtValue" xlink:label="Locator_us-gaap_LossContingencyDamagesSoughtValue_600" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LossContingenciesLineItems_599" xlink:to="Locator_us-gaap_LossContingencyDamagesSoughtValue_600" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_601" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingencyOpinionOfCounsel" xlink:label="Locator_us-gaap_LossContingencyOpinionOfCounsel_602" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LossContingenciesLineItems_601" xlink:to="Locator_us-gaap_LossContingencyOpinionOfCounsel_602" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_603" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_UnbilledChangeOrders" xlink:label="Locator_us-gaap_UnbilledChangeOrders_604" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LossContingenciesLineItems_603" xlink:to="Locator_us-gaap_UnbilledChangeOrders_604" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_605" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_ContractRevenueEarnedToDate" xlink:label="Locator_iesc_ContractRevenueEarnedToDate_606" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LossContingenciesLineItems_605" xlink:to="Locator_iesc_ContractRevenueEarnedToDate_606" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_607" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingencyAccrualAtCarryingValue" xlink:label="Locator_us-gaap_LossContingencyAccrualAtCarryingValue_608" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LossContingenciesLineItems_607" xlink:to="Locator_us-gaap_LossContingencyAccrualAtCarryingValue_608" order="6.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_609" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InsuranceSettlementsReceivableCurrent" xlink:label="Locator_us-gaap_InsuranceSettlementsReceivableCurrent_610" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LossContingenciesLineItems_609" xlink:to="Locator_us-gaap_InsuranceSettlementsReceivableCurrent_610" order="7.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_611" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingencyAccrualPayments" xlink:label="Locator_us-gaap_LossContingencyAccrualPayments_612" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LossContingenciesLineItems_611" xlink:to="Locator_us-gaap_LossContingencyAccrualPayments_612" order="8.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_613" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromLegalSettlements" xlink:label="Locator_us-gaap_ProceedsFromLegalSettlements_614" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LossContingenciesLineItems_613" xlink:to="Locator_us-gaap_ProceedsFromLegalSettlements_614" order="9.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="Locator_us-gaap_LossContingenciesLineItems_615" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_ProceedsPaymentContingent" xlink:label="Locator_iesc_ProceedsPaymentContingent_616" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_LossContingenciesLineItems_615" xlink:to="Locator_iesc_ProceedsPaymentContingent_616" order="10.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsLineItems" xlink:label="Locator_us-gaap_OtherCommitmentsLineItems_549" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsTable" xlink:label="Locator_us-gaap_OtherCommitmentsTable_550" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_OtherCommitmentsLineItems_549" xlink:to="Locator_us-gaap_OtherCommitmentsTable_550" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsTable" xlink:label="Locator_us-gaap_OtherCommitmentsTable_573" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsAxis" xlink:label="Locator_us-gaap_OtherCommitmentsAxis_574" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_OtherCommitmentsTable_573" xlink:to="Locator_us-gaap_OtherCommitmentsAxis_574" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsAxis" xlink:label="Locator_us-gaap_OtherCommitmentsAxis_579" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="Locator_us-gaap_OtherCommitmentsDomain_580" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_OtherCommitmentsAxis_579" xlink:to="Locator_us-gaap_OtherCommitmentsDomain_580" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsAxis" xlink:label="Locator_us-gaap_OtherCommitmentsAxis_621" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="Locator_us-gaap_OtherCommitmentsDomain_622" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_OtherCommitmentsAxis_621" xlink:to="Locator_us-gaap_OtherCommitmentsDomain_622" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="Locator_us-gaap_OtherCommitmentsDomain_581" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_InsuranceRelatedMember" xlink:label="Locator_iesc_InsuranceRelatedMember_582" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_OtherCommitmentsDomain_581" xlink:to="Locator_iesc_InsuranceRelatedMember_582" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="Locator_us-gaap_OtherCommitmentsDomain_583" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_VendorRelatedMember" xlink:label="Locator_iesc_VendorRelatedMember_584" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_OtherCommitmentsDomain_583" xlink:to="Locator_iesc_VendorRelatedMember_584" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherCommitmentsLineItems" xlink:label="Locator_us-gaap_OtherCommitmentsLineItems_595" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount" xlink:label="Locator_us-gaap_LettersOfCreditOutstandingAmount_596" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_OtherCommitmentsLineItems_595" xlink:to="Locator_us-gaap_LettersOfCreditOutstandingAmount_596" order="1.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/StockholdersEquityDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityClassOfTreasuryStockLineItems" xlink:label="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_697" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfTreasuryStockTable" xlink:label="Locator_us-gaap_ClassOfTreasuryStockTable_698" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_697" xlink:to="Locator_us-gaap_ClassOfTreasuryStockTable_698" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfTreasuryStockTable" xlink:label="Locator_us-gaap_ClassOfTreasuryStockTable_707" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis" xlink:label="Locator_us-gaap_ShareRepurchaseProgramAxis_708" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ClassOfTreasuryStockTable_707" xlink:to="Locator_us-gaap_ShareRepurchaseProgramAxis_708" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis" xlink:label="Locator_us-gaap_ShareRepurchaseProgramAxis_723" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="Locator_us-gaap_ShareRepurchaseProgramDomain_724" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_ShareRepurchaseProgramAxis_723" xlink:to="Locator_us-gaap_ShareRepurchaseProgramDomain_724" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis" xlink:label="Locator_us-gaap_ShareRepurchaseProgramAxis_775" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="Locator_us-gaap_ShareRepurchaseProgramDomain_776" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_ShareRepurchaseProgramAxis_775" xlink:to="Locator_us-gaap_ShareRepurchaseProgramDomain_776" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="Locator_us-gaap_ShareRepurchaseProgramDomain_725" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_The2015StockRepurchaseProgramMember" xlink:label="Locator_iesc_The2015StockRepurchaseProgramMember_726" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareRepurchaseProgramDomain_725" xlink:to="Locator_iesc_The2015StockRepurchaseProgramMember_726" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityClassOfTreasuryStockLineItems" xlink:label="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_761" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" xlink:label="Locator_us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased_762" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_761" xlink:to="Locator_us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased_762" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityClassOfTreasuryStockLineItems" xlink:label="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_763" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockRepurchaseProgramRemainingNumberOfSharesAuthorizedToBeRepurchased" xlink:label="Locator_us-gaap_StockRepurchaseProgramRemainingNumberOfSharesAuthorizedToBeRepurchased_764" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_763" xlink:to="Locator_us-gaap_StockRepurchaseProgramRemainingNumberOfSharesAuthorizedToBeRepurchased_764" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityClassOfTreasuryStockLineItems" xlink:label="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_765" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TreasuryStockSharesAcquired" xlink:label="Locator_us-gaap_TreasuryStockSharesAcquired_766" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_765" xlink:to="Locator_us-gaap_TreasuryStockSharesAcquired_766" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityClassOfTreasuryStockLineItems" xlink:label="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_767" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod" xlink:label="Locator_us-gaap_TreasuryStockValueAcquiredCostMethod_768" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_767" xlink:to="Locator_us-gaap_TreasuryStockValueAcquiredCostMethod_768" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityClassOfTreasuryStockLineItems" xlink:label="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_769" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharePrice" xlink:label="Locator_us-gaap_SharePrice_770" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_769" xlink:to="Locator_us-gaap_SharePrice_770" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityClassOfTreasuryStockLineItems" xlink:label="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_771" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare" xlink:label="Locator_us-gaap_TreasuryStockAcquiredAverageCostPerShare_772" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_EquityClassOfTreasuryStockLineItems_771" xlink:to="Locator_us-gaap_TreasuryStockAcquiredAverageCostPerShare_772" order="6.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_677" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="Locator_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_678" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_677" xlink:to="Locator_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_678" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="Locator_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_701" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="Locator_us-gaap_AwardTypeAxis_702" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_701" xlink:to="Locator_us-gaap_AwardTypeAxis_702" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="Locator_us-gaap_AwardTypeAxis_727" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_728" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_AwardTypeAxis_727" xlink:to="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_728" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="Locator_us-gaap_AwardTypeAxis_773" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_774" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_AwardTypeAxis_773" xlink:to="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_774" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_711" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedStockMember" xlink:label="Locator_us-gaap_RestrictedStockMember_712" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_711" xlink:to="Locator_us-gaap_RestrictedStockMember_712" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_713" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_PerformanceCashUnitsMember" xlink:label="Locator_iesc_PerformanceCashUnitsMember_714" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_713" xlink:to="Locator_iesc_PerformanceCashUnitsMember_714" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_715" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PhantomShareUnitsPSUsMember" xlink:label="Locator_us-gaap_PhantomShareUnitsPSUsMember_716" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_715" xlink:to="Locator_us-gaap_PhantomShareUnitsPSUsMember_716" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_717" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_PhantomshareunitspsusperformancebasedMember" xlink:label="Locator_iesc_PhantomshareunitspsusperformancebasedMember_718" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_717" xlink:to="Locator_iesc_PhantomshareunitspsusperformancebasedMember_718" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_719" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockOptionMember" xlink:label="Locator_us-gaap_StockOptionMember_720" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_719" xlink:to="Locator_us-gaap_StockOptionMember_720" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_721" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_UnrestrictedStockMember" xlink:label="Locator_iesc_UnrestrictedStockMember_722" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_721" xlink:to="Locator_iesc_UnrestrictedStockMember_722" order="6.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="Locator_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_703" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="Locator_us-gaap_PlanNameAxis_704" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_703" xlink:to="Locator_us-gaap_PlanNameAxis_704" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="Locator_us-gaap_PlanNameAxis_709" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="Locator_us-gaap_PlanNameDomain_710" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_PlanNameAxis_709" xlink:to="Locator_us-gaap_PlanNameDomain_710" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="Locator_us-gaap_PlanNameAxis_777" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="Locator_us-gaap_PlanNameDomain_778" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_PlanNameAxis_777" xlink:to="Locator_us-gaap_PlanNameDomain_778" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="Locator_us-gaap_PlanNameDomain_729" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_TwoThousandSixEquityIncentivePlanMember" xlink:label="Locator_iesc_TwoThousandSixEquityIncentivePlanMember_730" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_PlanNameDomain_729" xlink:to="Locator_iesc_TwoThousandSixEquityIncentivePlanMember_730" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="Locator_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_705" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="Locator_us-gaap_TitleOfIndividualAxis_706" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_705" xlink:to="Locator_us-gaap_TitleOfIndividualAxis_706" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="Locator_us-gaap_TitleOfIndividualAxis_731" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="Locator_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_732" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_TitleOfIndividualAxis_731" xlink:to="Locator_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_732" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="Locator_us-gaap_TitleOfIndividualAxis_779" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="Locator_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_780" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_TitleOfIndividualAxis_779" xlink:to="Locator_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_780" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="Locator_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_733" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DirectorMember" xlink:label="Locator_us-gaap_DirectorMember_734" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_733" xlink:to="Locator_us-gaap_DirectorMember_734" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_735" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="Locator_us-gaap_CommonStockSharesAuthorized_736" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_735" xlink:to="Locator_us-gaap_CommonStockSharesAuthorized_736" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_737" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="Locator_us-gaap_CommonStockSharesOutstanding_738" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_737" xlink:to="Locator_us-gaap_CommonStockSharesOutstanding_738" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_739" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation" xlink:label="Locator_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_740" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_739" xlink:to="Locator_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_740" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_741" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_742" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_741" xlink:to="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_742" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_743" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_744" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_743" xlink:to="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_744" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_745" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="Locator_us-gaap_AllocatedShareBasedCompensationExpense_746" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_745" xlink:to="Locator_us-gaap_AllocatedShareBasedCompensationExpense_746" order="6.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_747" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_RecognizedBenefit" xlink:label="Locator_iesc_RecognizedBenefit_748" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_747" xlink:to="Locator_iesc_RecognizedBenefit_748" order="7.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_749" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="Locator_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_750" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_749" xlink:to="Locator_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_750" order="8.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_751" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_AvailableCommonStockAuthorizedShares" xlink:label="Locator_iesc_AvailableCommonStockAuthorizedShares_752" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_751" xlink:to="Locator_iesc_AvailableCommonStockAuthorizedShares_752" order="9.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_753" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="Locator_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_754" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_753" xlink:to="Locator_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_754" order="10.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_755" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="Locator_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_756" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_755" xlink:to="Locator_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_756" order="11.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_757" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_758" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_757" xlink:to="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_758" order="12.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_759" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDescription" xlink:label="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDescription_760" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_759" xlink:to="Locator_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDescription_760" order="13.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/FairValueMeasurementsDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_839" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="Locator_us-gaap_FairValueInputsLevel1Member_840" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_839" xlink:to="Locator_us-gaap_FairValueInputsLevel1Member_840" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_841" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="Locator_us-gaap_FairValueInputsLevel2Member_842" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_841" xlink:to="Locator_us-gaap_FairValueInputsLevel2Member_842" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_843" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="Locator_us-gaap_FairValueInputsLevel3Member_844" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_843" xlink:to="Locator_us-gaap_FairValueInputsLevel3Member_844" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_831" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_832" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_831" xlink:to="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_832" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_835" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="Locator_us-gaap_FairValueByFairValueHierarchyLevelAxis_836" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_835" xlink:to="Locator_us-gaap_FairValueByFairValueHierarchyLevelAxis_836" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="Locator_us-gaap_FairValueByFairValueHierarchyLevelAxis_837" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_838" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_FairValueByFairValueHierarchyLevelAxis_837" xlink:to="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_838" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="Locator_us-gaap_FairValueByFairValueHierarchyLevelAxis_853" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_854" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_FairValueByFairValueHierarchyLevelAxis_853" xlink:to="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_854" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_845" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetsFairValueDisclosure" xlink:label="Locator_us-gaap_AssetsFairValueDisclosure_846" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_845" xlink:to="Locator_us-gaap_AssetsFairValueDisclosure_846" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_847" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LiabilitiesFairValueDisclosure" xlink:label="Locator_us-gaap_LiabilitiesFairValueDisclosure_848" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_847" xlink:to="Locator_us-gaap_LiabilitiesFairValueDisclosure_848" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_849" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent" xlink:label="Locator_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_850" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_849" xlink:to="Locator_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_850" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_851" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueNetAssetLiability" xlink:label="Locator_us-gaap_FairValueNetAssetLiability_852" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_851" xlink:to="Locator_us-gaap_FairValueNetAssetLiability_852" order="4.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/GoodwilAndIntangibleAssetsOtherIntangibleAssetsDetails">
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_IntangibleAssetsLineItems" xlink:label="Locator_iesc_IntangibleAssetsLineItems_889" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_IntangibleassetstableTable" xlink:label="Locator_iesc_IntangibleassetstableTable_890" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_iesc_IntangibleAssetsLineItems_889" xlink:to="Locator_iesc_IntangibleassetstableTable_890" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_IntangibleassetstableTable" xlink:label="Locator_iesc_IntangibleassetstableTable_893" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_894" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_iesc_IntangibleassetstableTable_893" xlink:to="Locator_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_894" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_897" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_898" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_897" xlink:to="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_898" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_925" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_926" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_925" xlink:to="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_926" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_899" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TrademarksAndTradeNamesMember" xlink:label="Locator_us-gaap_TrademarksAndTradeNamesMember_900" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_899" xlink:to="Locator_us-gaap_TrademarksAndTradeNamesMember_900" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_901" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherIntangibleAssetsMember" xlink:label="Locator_us-gaap_OtherIntangibleAssetsMember_902" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_901" xlink:to="Locator_us-gaap_OtherIntangibleAssetsMember_902" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_903" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="Locator_us-gaap_CustomerRelationshipsMember_904" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_903" xlink:to="Locator_us-gaap_CustomerRelationshipsMember_904" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_905" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember" xlink:label="Locator_us-gaap_DevelopedTechnologyRightsMember_906" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_905" xlink:to="Locator_us-gaap_DevelopedTechnologyRightsMember_906" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_907" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OrderOrProductionBacklogMember" xlink:label="Locator_us-gaap_OrderOrProductionBacklogMember_908" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_907" xlink:to="Locator_us-gaap_OrderOrProductionBacklogMember_908" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_909" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_OffMarketPocAdjustmentMember" xlink:label="Locator_iesc_OffMarketPocAdjustmentMember_910" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_909" xlink:to="Locator_iesc_OffMarketPocAdjustmentMember_910" order="6.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_IntangibleassetstableTable" xlink:label="Locator_iesc_IntangibleassetstableTable_895" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="Locator_srt_RangeAxis_896" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_iesc_IntangibleassetstableTable_895" xlink:to="Locator_srt_RangeAxis_896" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="Locator_srt_RangeAxis_911" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="Locator_srt_RangeMember_912" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_srt_RangeAxis_911" xlink:to="Locator_srt_RangeMember_912" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="Locator_srt_RangeAxis_927" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="Locator_srt_RangeMember_928" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_srt_RangeAxis_927" xlink:to="Locator_srt_RangeMember_928" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="Locator_srt_RangeMember_913" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="Locator_srt_MaximumMember_914" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_srt_RangeMember_913" xlink:to="Locator_srt_MaximumMember_914" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="Locator_srt_RangeMember_915" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="Locator_srt_MinimumMember_916" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_srt_RangeMember_915" xlink:to="Locator_srt_MinimumMember_916" order="2.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_IntangibleAssetsLineItems" xlink:label="Locator_iesc_IntangibleAssetsLineItems_917" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetUsefulLife_918" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_IntangibleAssetsLineItems_917" xlink:to="Locator_us-gaap_FiniteLivedIntangibleAssetUsefulLife_918" order="1.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_IntangibleAssetsLineItems" xlink:label="Locator_iesc_IntangibleAssetsLineItems_919" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill" xlink:label="Locator_us-gaap_IntangibleAssetsGrossExcludingGoodwill_920" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_IntangibleAssetsLineItems_919" xlink:to="Locator_us-gaap_IntangibleAssetsGrossExcludingGoodwill_920" order="2.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_IntangibleAssetsLineItems" xlink:label="Locator_iesc_IntangibleAssetsLineItems_921" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="Locator_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_922" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_IntangibleAssetsLineItems_921" xlink:to="Locator_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_922" order="3.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_IntangibleAssetsLineItems" xlink:label="Locator_iesc_IntangibleAssetsLineItems_923" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="Locator_us-gaap_IntangibleAssetsNetExcludingGoodwill_924" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_IntangibleAssetsLineItems_923" xlink:to="Locator_us-gaap_IntangibleAssetsNetExcludingGoodwill_924" order="4.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/ControllingShareholderDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="Locator_us-gaap_RelatedPartyDomain_983" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MajorityShareholderMember" xlink:label="Locator_us-gaap_MajorityShareholderMember_984" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_RelatedPartyDomain_983" xlink:to="Locator_us-gaap_MajorityShareholderMember_984" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="Locator_us-gaap_RelatedPartyTransactionLineItems_977" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="Locator_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_978" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_RelatedPartyTransactionLineItems_977" xlink:to="Locator_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_978" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="Locator_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_979" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="Locator_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_980" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_979" xlink:to="Locator_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_980" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="Locator_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_981" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="Locator_us-gaap_RelatedPartyDomain_982" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_981" xlink:to="Locator_us-gaap_RelatedPartyDomain_982" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="Locator_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_991" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="Locator_us-gaap_RelatedPartyDomain_992" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_991" xlink:to="Locator_us-gaap_RelatedPartyDomain_992" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="Locator_us-gaap_RelatedPartyTransactionLineItems_985" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionDescriptionOfTransaction" xlink:label="Locator_us-gaap_RelatedPartyTransactionDescriptionOfTransaction_986" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_RelatedPartyTransactionLineItems_985" xlink:to="Locator_us-gaap_RelatedPartyTransactionDescriptionOfTransaction_986" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="Locator_us-gaap_RelatedPartyTransactionLineItems_987" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LeaseAndRentalExpense" xlink:label="Locator_us-gaap_LeaseAndRentalExpense_988" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_RelatedPartyTransactionLineItems_987" xlink:to="Locator_us-gaap_LeaseAndRentalExpense_988" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="Locator_us-gaap_RelatedPartyTransactionLineItems_989" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_OwnershipPercentageOfCommonStock" xlink:label="Locator_iesc_OwnershipPercentageOfCommonStock_990" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_RelatedPartyTransactionLineItems_989" xlink:to="Locator_iesc_OwnershipPercentageOfCommonStock_990" order="3.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/BusinessCombinationsDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1057" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommercialIndustrialMember" xlink:label="Locator_iesc_CommercialIndustrialMember_1058" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1057" xlink:to="Locator_iesc_CommercialIndustrialMember_1058" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1059" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommunicationsMember" xlink:label="Locator_iesc_CommunicationsMember_1060" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1059" xlink:to="Locator_iesc_CommunicationsMember_1060" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1055" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_InfrastructureSolutionsMember" xlink:label="Locator_iesc_InfrastructureSolutionsMember_1056" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1055" xlink:to="Locator_iesc_InfrastructureSolutionsMember_1056" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1061" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NumberOfBusinessesAcquired" xlink:label="Locator_us-gaap_NumberOfBusinessesAcquired_1062" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1061" xlink:to="Locator_us-gaap_NumberOfBusinessesAcquired_1062" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1063" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionNameOfAcquiredEntity" xlink:label="Locator_us-gaap_BusinessAcquisitionNameOfAcquiredEntity_1064" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1063" xlink:to="Locator_us-gaap_BusinessAcquisitionNameOfAcquiredEntity_1064" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1065" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionDescriptionOfAcquiredEntity" xlink:label="Locator_us-gaap_BusinessAcquisitionDescriptionOfAcquiredEntity_1066" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1065" xlink:to="Locator_us-gaap_BusinessAcquisitionDescriptionOfAcquiredEntity_1066" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1067" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionDateOfAcquisitionAgreement1" xlink:label="Locator_us-gaap_BusinessAcquisitionDateOfAcquisitionAgreement1_1068" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1067" xlink:to="Locator_us-gaap_BusinessAcquisitionDateOfAcquisitionAgreement1_1068" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1071" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1" xlink:label="Locator_us-gaap_BusinessCombinationConsiderationTransferred1_1072" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1071" xlink:to="Locator_us-gaap_BusinessCombinationConsiderationTransferred1_1072" order="6.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1073" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="Locator_us-gaap_PaymentsToAcquireBusinessesGross_1074" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1073" xlink:to="Locator_us-gaap_PaymentsToAcquireBusinessesGross_1074" order="7.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1075" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred" xlink:label="Locator_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_1076" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1075" xlink:to="Locator_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_1076" order="8.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1077" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent" xlink:label="Locator_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_1078" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1077" xlink:to="Locator_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_1078" order="9.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1079" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1" xlink:label="Locator_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_1080" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1079" xlink:to="Locator_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_1080" order="10.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1081" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount" xlink:label="Locator_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_1082" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1081" xlink:to="Locator_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_1082" order="11.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1083" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual" xlink:label="Locator_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_1084" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1083" xlink:to="Locator_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_1084" order="12.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1085" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual" xlink:label="Locator_us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual_1086" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1085" xlink:to="Locator_us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual_1086" order="13.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1069" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent" xlink:label="Locator_us-gaap_MinorityInterestOwnershipPercentageByParent_1070" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1069" xlink:to="Locator_us-gaap_MinorityInterestOwnershipPercentageByParent_1070" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1035" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1036" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1035" xlink:to="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1036" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1039" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_1040" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1039" xlink:to="Locator_us-gaap_StatementBusinessSegmentsAxis_1040" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_1053" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1054" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_StatementBusinessSegmentsAxis_1053" xlink:to="Locator_us-gaap_SegmentDomain_1054" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1037" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="Locator_us-gaap_BusinessAcquisitionAxis_1038" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1037" xlink:to="Locator_us-gaap_BusinessAcquisitionAxis_1038" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="Locator_us-gaap_BusinessAcquisitionAxis_1041" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1042" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_BusinessAcquisitionAxis_1041" xlink:to="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1042" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="Locator_us-gaap_BusinessAcquisitionAxis_1087" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1088" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_BusinessAcquisitionAxis_1087" xlink:to="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1088" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1047" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_FreemanAcquisitionMember" xlink:label="Locator_iesc_FreemanAcquisitionMember_1048" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1047" xlink:to="Locator_iesc_FreemanAcquisitionMember_1048" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1049" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_BusinessacquisitiontechnicalservicesmemberMember" xlink:label="Locator_iesc_BusinessacquisitiontechnicalservicesmemberMember_1050" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1049" xlink:to="Locator_iesc_BusinessacquisitiontechnicalservicesmemberMember_1050" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1051" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_BusinessacquisitionnextmemberMember" xlink:label="Locator_iesc_BusinessacquisitionnextmemberMember_1052" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1051" xlink:to="Locator_iesc_BusinessacquisitionnextmemberMember_1052" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1043" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_SeriesOfBusinessAcquisitions2017MemberMember" xlink:label="Locator_iesc_SeriesOfBusinessAcquisitions2017MemberMember_1044" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1043" xlink:to="Locator_iesc_SeriesOfBusinessAcquisitions2017MemberMember_1044" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1045" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_Seriesofbusinessacquisitions2018membermemberMember" xlink:label="Locator_iesc_Seriesofbusinessacquisitions2018membermemberMember_1046" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1045" xlink:to="Locator_iesc_Seriesofbusinessacquisitions2018membermemberMember_1046" order="2.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/BusinessCombinationsProFormasDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1131" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue" xlink:label="Locator_us-gaap_BusinessAcquisitionsProFormaRevenue_1132" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1131" xlink:to="Locator_us-gaap_BusinessAcquisitionsProFormaRevenue_1132" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1133" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaIncomeLossFromContinuingOperationsBeforeChangesInAccountingAndExtraordinaryItemsNetOfTax" xlink:label="Locator_us-gaap_BusinessAcquisitionsProFormaIncomeLossFromContinuingOperationsBeforeChangesInAccountingAndExtraordinaryItemsNetOfTax_1134" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1133" xlink:to="Locator_us-gaap_BusinessAcquisitionsProFormaIncomeLossFromContinuingOperationsBeforeChangesInAccountingAndExtraordinaryItemsNetOfTax_1134" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1121" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1122" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1121" xlink:to="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1122" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1125" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="Locator_us-gaap_BusinessAcquisitionAxis_1126" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1125" xlink:to="Locator_us-gaap_BusinessAcquisitionAxis_1126" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="Locator_us-gaap_BusinessAcquisitionAxis_1127" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1128" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_BusinessAcquisitionAxis_1127" xlink:to="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1128" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1129" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_SeriesOfBusinessAcquisitions2017MemberMember" xlink:label="Locator_iesc_SeriesOfBusinessAcquisitions2017MemberMember_1130" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1129" xlink:to="Locator_iesc_SeriesOfBusinessAcquisitions2017MemberMember_1130" order="1.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/BusinessDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1173" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommercialIndustrialMember" xlink:label="Locator_iesc_CommercialIndustrialMember_1174" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1173" xlink:to="Locator_iesc_CommercialIndustrialMember_1174" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1175" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent" xlink:label="Locator_us-gaap_MinorityInterestOwnershipPercentageByParent_1176" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1175" xlink:to="Locator_us-gaap_MinorityInterestOwnershipPercentageByParent_1176" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1177" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" xlink:label="Locator_us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners_1178" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1177" xlink:to="Locator_us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners_1178" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1179" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RedeemableNoncontrollingInterestEquityRedemptionValue" xlink:label="Locator_us-gaap_RedeemableNoncontrollingInterestEquityRedemptionValue_1180" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1179" xlink:to="Locator_us-gaap_RedeemableNoncontrollingInterestEquityRedemptionValue_1180" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1181" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MinorityInterestChangeInRedemptionValue" xlink:label="Locator_us-gaap_MinorityInterestChangeInRedemptionValue_1182" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1181" xlink:to="Locator_us-gaap_MinorityInterestChangeInRedemptionValue_1182" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1163" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1164" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1163" xlink:to="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1164" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1169" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_1170" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1169" xlink:to="Locator_us-gaap_StatementBusinessSegmentsAxis_1170" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_1171" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1172" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_StatementBusinessSegmentsAxis_1171" xlink:to="Locator_us-gaap_SegmentDomain_1172" order="1.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/SubsequentEventsDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="Locator_us-gaap_BusinessAcquisitionAxis_1217" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1218" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_BusinessAcquisitionAxis_1217" xlink:to="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1218" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1219" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_NextBusinessAcquisitionMember" xlink:label="Locator_iesc_NextBusinessAcquisitionMember_1220" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1219" xlink:to="Locator_iesc_NextBusinessAcquisitionMember_1220" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="Locator_us-gaap_CreditFacilityAxis_1213" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="Locator_us-gaap_CreditFacilityDomain_1214" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_CreditFacilityAxis_1213" xlink:to="Locator_us-gaap_CreditFacilityDomain_1214" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="Locator_us-gaap_CreditFacilityDomain_1215" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="Locator_us-gaap_RevolvingCreditFacilityMember_1216" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_CreditFacilityDomain_1215" xlink:to="Locator_us-gaap_RevolvingCreditFacilityMember_1216" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventLineItems" xlink:label="Locator_us-gaap_SubsequentEventLineItems_1201" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTable" xlink:label="Locator_us-gaap_SubsequentEventTable_1202" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_SubsequentEventLineItems_1201" xlink:to="Locator_us-gaap_SubsequentEventTable_1202" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTable" xlink:label="Locator_us-gaap_SubsequentEventTable_1203" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="Locator_us-gaap_SubsequentEventTypeAxis_1204" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_SubsequentEventTable_1203" xlink:to="Locator_us-gaap_SubsequentEventTypeAxis_1204" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="Locator_us-gaap_SubsequentEventTypeAxis_1209" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="Locator_us-gaap_SubsequentEventTypeDomain_1210" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_SubsequentEventTypeAxis_1209" xlink:to="Locator_us-gaap_SubsequentEventTypeDomain_1210" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="Locator_us-gaap_SubsequentEventTypeAxis_1229" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="Locator_us-gaap_SubsequentEventTypeDomain_1230" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_SubsequentEventTypeAxis_1229" xlink:to="Locator_us-gaap_SubsequentEventTypeDomain_1230" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="Locator_us-gaap_SubsequentEventTypeDomain_1211" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="Locator_us-gaap_SubsequentEventMember_1212" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SubsequentEventTypeDomain_1211" xlink:to="Locator_us-gaap_SubsequentEventMember_1212" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTable" xlink:label="Locator_us-gaap_SubsequentEventTable_1205" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="Locator_us-gaap_CreditFacilityAxis_1206" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_SubsequentEventTable_1205" xlink:to="Locator_us-gaap_CreditFacilityAxis_1206" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventTable" xlink:label="Locator_us-gaap_SubsequentEventTable_1207" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="Locator_us-gaap_BusinessAcquisitionAxis_1208" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_SubsequentEventTable_1207" xlink:to="Locator_us-gaap_BusinessAcquisitionAxis_1208" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventLineItems" xlink:label="Locator_us-gaap_SubsequentEventLineItems_1221" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventDescription" xlink:label="Locator_us-gaap_SubsequentEventDescription_1222" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SubsequentEventLineItems_1221" xlink:to="Locator_us-gaap_SubsequentEventDescription_1222" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventLineItems" xlink:label="Locator_us-gaap_SubsequentEventLineItems_1223" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityCovenantTerms" xlink:label="Locator_us-gaap_LineOfCreditFacilityCovenantTerms_1224" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SubsequentEventLineItems_1223" xlink:to="Locator_us-gaap_LineOfCreditFacilityCovenantTerms_1224" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventLineItems" xlink:label="Locator_us-gaap_SubsequentEventLineItems_1225" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionNameOfAcquiredEntity" xlink:label="Locator_us-gaap_BusinessAcquisitionNameOfAcquiredEntity_1226" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SubsequentEventLineItems_1225" xlink:to="Locator_us-gaap_BusinessAcquisitionNameOfAcquiredEntity_1226" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsequentEventLineItems" xlink:label="Locator_us-gaap_SubsequentEventLineItems_1227" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionDescriptionOfAcquiredEntity" xlink:label="Locator_us-gaap_BusinessAcquisitionDescriptionOfAcquiredEntity_1228" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SubsequentEventLineItems_1227" xlink:to="Locator_us-gaap_BusinessAcquisitionDescriptionOfAcquiredEntity_1228" order="4.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/FairValueMeasurementsUnobservableInputsDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1273" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="Locator_us-gaap_FairValueInputsLevel3Member_1274" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1273" xlink:to="Locator_us-gaap_FairValueInputsLevel3Member_1274" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="Locator_us-gaap_FairValueByFairValueHierarchyLevelAxis_1271" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1272" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_FairValueByFairValueHierarchyLevelAxis_1271" xlink:to="Locator_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1272" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1261" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1262" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1261" xlink:to="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1262" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1265" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="Locator_us-gaap_FairValueByLiabilityClassAxis_1266" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1265" xlink:to="Locator_us-gaap_FairValueByLiabilityClassAxis_1266" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="Locator_us-gaap_FairValueByLiabilityClassAxis_1275" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_1276" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_FairValueByLiabilityClassAxis_1275" xlink:to="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_1276" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="Locator_us-gaap_FairValueByLiabilityClassAxis_1285" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_1286" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_FairValueByLiabilityClassAxis_1285" xlink:to="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_1286" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_1269" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_ContingentconsiderationMember" xlink:label="Locator_iesc_ContingentconsiderationMember_1270" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_1269" xlink:to="Locator_iesc_ContingentconsiderationMember_1270" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1267" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="Locator_us-gaap_FairValueByFairValueHierarchyLevelAxis_1268" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_1267" xlink:to="Locator_us-gaap_FairValueByFairValueHierarchyLevelAxis_1268" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1277" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue" xlink:label="Locator_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_1278" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1277" xlink:to="Locator_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_1278" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1279" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues" xlink:label="Locator_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues_1280" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1279" xlink:to="Locator_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues_1280" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1281" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements" xlink:label="Locator_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_1282" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1281" xlink:to="Locator_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_1282" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:label="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1283" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings" xlink:label="Locator_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_1284" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_1283" xlink:to="Locator_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_1284" order="4.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/GoodwillAndIntangibleAssetsGoodwillRollForwardDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1319" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommercialIndustrialMember" xlink:label="Locator_iesc_CommercialIndustrialMember_1320" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1319" xlink:to="Locator_iesc_CommercialIndustrialMember_1320" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1321" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommunicationsMember" xlink:label="Locator_iesc_CommunicationsMember_1322" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1321" xlink:to="Locator_iesc_CommunicationsMember_1322" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1323" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_InfrastructureSolutionsMember" xlink:label="Locator_iesc_InfrastructureSolutionsMember_1324" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1323" xlink:to="Locator_iesc_InfrastructureSolutionsMember_1324" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1325" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_ResidentialMember" xlink:label="Locator_iesc_ResidentialMember_1326" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1325" xlink:to="Locator_iesc_ResidentialMember_1326" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_1317" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1318" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_StatementBusinessSegmentsAxis_1317" xlink:to="Locator_us-gaap_SegmentDomain_1318" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillLineItems" xlink:label="Locator_us-gaap_GoodwillLineItems_1313" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfGoodwillTable" xlink:label="Locator_us-gaap_ScheduleOfGoodwillTable_1314" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_GoodwillLineItems_1313" xlink:to="Locator_us-gaap_ScheduleOfGoodwillTable_1314" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfGoodwillTable" xlink:label="Locator_us-gaap_ScheduleOfGoodwillTable_1315" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_1316" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfGoodwillTable_1315" xlink:to="Locator_us-gaap_StatementBusinessSegmentsAxis_1316" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillLineItems" xlink:label="Locator_us-gaap_GoodwillLineItems_1327" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Goodwill" xlink:label="Locator_us-gaap_Goodwill_1328" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_GoodwillLineItems_1327" xlink:to="Locator_us-gaap_Goodwill_1328" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillLineItems" xlink:label="Locator_us-gaap_GoodwillLineItems_1329" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="Locator_us-gaap_GoodwillAcquiredDuringPeriod_1330" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_GoodwillLineItems_1329" xlink:to="Locator_us-gaap_GoodwillAcquiredDuringPeriod_1330" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillLineItems" xlink:label="Locator_us-gaap_GoodwillLineItems_1331" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit" xlink:label="Locator_us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit_1332" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_GoodwillLineItems_1331" xlink:to="Locator_us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit_1332" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillLineItems" xlink:label="Locator_us-gaap_GoodwillLineItems_1333" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GoodwillPurchaseAccountingAdjustments" xlink:label="Locator_us-gaap_GoodwillPurchaseAccountingAdjustments_1334" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_GoodwillLineItems_1333" xlink:to="Locator_us-gaap_GoodwillPurchaseAccountingAdjustments_1334" order="4.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/BusinessCombinationsAssetsDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1367" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1368" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1367" xlink:to="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1368" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1383" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="Locator_us-gaap_BusinessAcquisitionAxis_1384" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_1383" xlink:to="Locator_us-gaap_BusinessAcquisitionAxis_1384" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="Locator_us-gaap_BusinessAcquisitionAxis_1387" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1388" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_BusinessAcquisitionAxis_1387" xlink:to="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1388" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1385" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_Seriesofbusinessacquisitions2018membermemberMember" xlink:label="Locator_iesc_Seriesofbusinessacquisitions2018membermemberMember_1386" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionAcquireeDomain_1385" xlink:to="Locator_iesc_Seriesofbusinessacquisitions2018membermemberMember_1386" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1389" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetsCurrent" xlink:label="Locator_us-gaap_AssetsCurrent_1390" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1389" xlink:to="Locator_us-gaap_AssetsCurrent_1390" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1391" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="Locator_us-gaap_PropertyPlantAndEquipmentNet_1392" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1391" xlink:to="Locator_us-gaap_PropertyPlantAndEquipmentNet_1392" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1393" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="Locator_us-gaap_IntangibleAssetsNetExcludingGoodwill_1394" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1393" xlink:to="Locator_us-gaap_IntangibleAssetsNetExcludingGoodwill_1394" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1395" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Goodwill" xlink:label="Locator_us-gaap_Goodwill_1396" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1395" xlink:to="Locator_us-gaap_Goodwill_1396" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1397" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LiabilitiesCurrent" xlink:label="Locator_us-gaap_LiabilitiesCurrent_1398" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1397" xlink:to="Locator_us-gaap_LiabilitiesCurrent_1398" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1399" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LiabilitiesNoncurrent" xlink:label="Locator_us-gaap_LiabilitiesNoncurrent_1400" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1399" xlink:to="Locator_us-gaap_LiabilitiesNoncurrent_1400" order="6.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1401" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxLiabilities" xlink:label="Locator_us-gaap_DeferredTaxLiabilities_1402" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1401" xlink:to="Locator_us-gaap_DeferredTaxLiabilities_1402" order="7.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="Locator_us-gaap_BusinessAcquisitionLineItems_1403" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet" xlink:label="Locator_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_1404" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_BusinessAcquisitionLineItems_1403" xlink:to="Locator_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_1404" order="8.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/ConsolidatedStatementOfStockholdersEquity">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1443" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="Locator_us-gaap_StatementTable_1444" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_StatementLineItems_1443" xlink:to="Locator_us-gaap_StatementTable_1444" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="Locator_us-gaap_StatementTable_1445" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="Locator_us-gaap_StatementEquityComponentsAxis_1446" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_StatementTable_1445" xlink:to="Locator_us-gaap_StatementEquityComponentsAxis_1446" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="Locator_us-gaap_StatementEquityComponentsAxis_1447" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="Locator_us-gaap_EquityComponentDomain_1448" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_StatementEquityComponentsAxis_1447" xlink:to="Locator_us-gaap_EquityComponentDomain_1448" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="Locator_us-gaap_StatementEquityComponentsAxis_1485" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="Locator_us-gaap_EquityComponentDomain_1486" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_StatementEquityComponentsAxis_1485" xlink:to="Locator_us-gaap_EquityComponentDomain_1486" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="Locator_us-gaap_EquityComponentDomain_1449" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ParentMember" xlink:label="Locator_us-gaap_ParentMember_1450" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_EquityComponentDomain_1449" xlink:to="Locator_us-gaap_ParentMember_1450" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ParentMember" xlink:label="Locator_us-gaap_ParentMember_1451" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockMember" xlink:label="Locator_us-gaap_CommonStockMember_1452" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ParentMember_1451" xlink:to="Locator_us-gaap_CommonStockMember_1452" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ParentMember" xlink:label="Locator_us-gaap_ParentMember_1453" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TreasuryStockMember" xlink:label="Locator_us-gaap_TreasuryStockMember_1454" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ParentMember_1453" xlink:to="Locator_us-gaap_TreasuryStockMember_1454" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ParentMember" xlink:label="Locator_us-gaap_ParentMember_1455" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="Locator_us-gaap_AdditionalPaidInCapitalMember_1456" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ParentMember_1455" xlink:to="Locator_us-gaap_AdditionalPaidInCapitalMember_1456" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ParentMember" xlink:label="Locator_us-gaap_ParentMember_1457" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="Locator_us-gaap_AccumulatedOtherComprehensiveIncomeMember_1458" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ParentMember_1457" xlink:to="Locator_us-gaap_AccumulatedOtherComprehensiveIncomeMember_1458" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ParentMember" xlink:label="Locator_us-gaap_ParentMember_1459" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="Locator_us-gaap_RetainedEarningsMember_1460" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ParentMember_1459" xlink:to="Locator_us-gaap_RetainedEarningsMember_1460" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1461" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="Locator_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_1462" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1461" xlink:to="Locator_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_1462" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1463" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharesIssued" xlink:label="Locator_us-gaap_SharesIssued_1464" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1463" xlink:to="Locator_us-gaap_SharesIssued_1464" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1465" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures" xlink:label="Locator_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures_1466" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1465" xlink:to="Locator_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures_1466" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1467" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures" xlink:label="Locator_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_1468" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1467" xlink:to="Locator_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_1468" order="4.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1469" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption" xlink:label="Locator_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_1470" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1469" xlink:to="Locator_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_1470" order="5.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1471" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod" xlink:label="Locator_us-gaap_TreasuryStockValueAcquiredCostMethod_1472" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1471" xlink:to="Locator_us-gaap_TreasuryStockValueAcquiredCostMethod_1472" order="6.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1473" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TreasuryStockSharesAcquired" xlink:label="Locator_us-gaap_TreasuryStockSharesAcquired_1474" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1473" xlink:to="Locator_us-gaap_TreasuryStockSharesAcquired_1474" order="7.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1475" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="Locator_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_1476" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1475" xlink:to="Locator_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_1476" order="8.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1477" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="Locator_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_1478" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1477" xlink:to="Locator_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_1478" order="9.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1479" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="Locator_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_1480" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1479" xlink:to="Locator_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_1480" order="10.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1481" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_Noncontrollinginterestincreasefromfairvalueadjustment" xlink:label="Locator_iesc_Noncontrollinginterestincreasefromfairvalueadjustment_1482" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1481" xlink:to="Locator_iesc_Noncontrollinginterestincreasefromfairvalueadjustment_1482" order="11.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="Locator_us-gaap_StatementLineItems_1483" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="Locator_us-gaap_NetIncomeLoss_1484" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_StatementLineItems_1483" xlink:to="Locator_us-gaap_NetIncomeLoss_1484" order="12.0" />
    </definitionLink>
    <definitionLink xlink:type="extended" xlink:role="http://www.ies-co.com/role/RevenueRecognitionDetails">
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1541" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommercialIndustrialMember" xlink:label="Locator_iesc_CommercialIndustrialMember_1542" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1541" xlink:to="Locator_iesc_CommercialIndustrialMember_1542" order="1.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommercialIndustrialMember" xlink:label="Locator_iesc_CommercialIndustrialMember_1551" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommercialMemberMember" xlink:label="Locator_iesc_CommercialMemberMember_1552" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_CommercialIndustrialMember_1551" xlink:to="Locator_iesc_CommercialMemberMember_1552" order="1.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommercialIndustrialMember" xlink:label="Locator_iesc_CommercialIndustrialMember_1553" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_IndustrialMemberMember" xlink:label="Locator_iesc_IndustrialMemberMember_1554" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_CommercialIndustrialMember_1553" xlink:to="Locator_iesc_IndustrialMemberMember_1554" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1543" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CommunicationsMember" xlink:label="Locator_iesc_CommunicationsMember_1544" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1543" xlink:to="Locator_iesc_CommunicationsMember_1544" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1545" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_InfrastructureSolutionsMember" xlink:label="Locator_iesc_InfrastructureSolutionsMember_1546" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1545" xlink:to="Locator_iesc_InfrastructureSolutionsMember_1546" order="3.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_InfrastructureSolutionsMember" xlink:label="Locator_iesc_InfrastructureSolutionsMember_1555" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_IndustrialServicesMemberMember" xlink:label="Locator_iesc_IndustrialServicesMemberMember_1556" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_InfrastructureSolutionsMember_1555" xlink:to="Locator_iesc_IndustrialServicesMemberMember_1556" order="1.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_InfrastructureSolutionsMember" xlink:label="Locator_iesc_InfrastructureSolutionsMember_1557" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_CustomPowerSolutionsMemberMember" xlink:label="Locator_iesc_CustomPowerSolutionsMemberMember_1558" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_InfrastructureSolutionsMember_1557" xlink:to="Locator_iesc_CustomPowerSolutionsMemberMember_1558" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1547" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_ResidentialMember" xlink:label="Locator_iesc_ResidentialMember_1548" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_SegmentDomain_1547" xlink:to="Locator_iesc_ResidentialMember_1548" order="4.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_ResidentialMember" xlink:label="Locator_iesc_ResidentialMember_1559" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_SingleFamilyMemberMember" xlink:label="Locator_iesc_SingleFamilyMemberMember_1560" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_ResidentialMember_1559" xlink:to="Locator_iesc_SingleFamilyMemberMember_1560" order="1.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_ResidentialMember" xlink:label="Locator_iesc_ResidentialMember_1561" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_MultiFamilyMemberMember" xlink:label="Locator_iesc_MultiFamilyMemberMember_1562" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_ResidentialMember_1561" xlink:to="Locator_iesc_MultiFamilyMemberMember_1562" order="2.0" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_ResidentialMember" xlink:label="Locator_iesc_ResidentialMember_1563" />
        <loc xlink:type="locator" xlink:href="iesc-20181231.xsd#iesc_OtherMemberMember" xlink:label="Locator_iesc_OtherMemberMember_1564" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_iesc_ResidentialMember_1563" xlink:to="Locator_iesc_OtherMemberMember_1564" order="3.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_1549" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SegmentDomain" xlink:label="Locator_us-gaap_SegmentDomain_1550" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_StatementBusinessSegmentsAxis_1549" xlink:to="Locator_us-gaap_SegmentDomain_1550" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="Locator_us-gaap_DisaggregationOfRevenueLineItems_1535" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="Locator_us-gaap_DisaggregationOfRevenueTable_1536" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="Locator_us-gaap_DisaggregationOfRevenueLineItems_1535" xlink:to="Locator_us-gaap_DisaggregationOfRevenueTable_1536" order="-1.0" xbrldt:contextElement="segment" xbrldt:closed="true" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="Locator_us-gaap_DisaggregationOfRevenueTable_1537" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="Locator_us-gaap_StatementBusinessSegmentsAxis_1538" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_DisaggregationOfRevenueTable_1537" xlink:to="Locator_us-gaap_StatementBusinessSegmentsAxis_1538" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="Locator_us-gaap_DisaggregationOfRevenueTable_1539" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ContractWithCustomerDurationAxis" xlink:label="Locator_us-gaap_ContractWithCustomerDurationAxis_1540" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="Locator_us-gaap_DisaggregationOfRevenueTable_1539" xlink:to="Locator_us-gaap_ContractWithCustomerDurationAxis_1540" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ContractWithCustomerDurationAxis" xlink:label="Locator_us-gaap_ContractWithCustomerDurationAxis_1565" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ContractWithCustomerDurationDomain" xlink:label="Locator_us-gaap_ContractWithCustomerDurationDomain_1566" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="Locator_us-gaap_ContractWithCustomerDurationAxis_1565" xlink:to="Locator_us-gaap_ContractWithCustomerDurationDomain_1566" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ContractWithCustomerDurationAxis" xlink:label="Locator_us-gaap_ContractWithCustomerDurationAxis_1573" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ContractWithCustomerDurationDomain" xlink:label="Locator_us-gaap_ContractWithCustomerDurationDomain_1574" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="Locator_us-gaap_ContractWithCustomerDurationAxis_1573" xlink:to="Locator_us-gaap_ContractWithCustomerDurationDomain_1574" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ContractWithCustomerDurationDomain" xlink:label="Locator_us-gaap_ContractWithCustomerDurationDomain_1567" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FixedPriceContractMember" xlink:label="Locator_us-gaap_FixedPriceContractMember_1568" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ContractWithCustomerDurationDomain_1567" xlink:to="Locator_us-gaap_FixedPriceContractMember_1568" order="1.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ContractWithCustomerDurationDomain" xlink:label="Locator_us-gaap_ContractWithCustomerDurationDomain_1569" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TimeAndMaterialsContractMember" xlink:label="Locator_us-gaap_TimeAndMaterialsContractMember_1570" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_ContractWithCustomerDurationDomain_1569" xlink:to="Locator_us-gaap_TimeAndMaterialsContractMember_1570" order="2.0" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="Locator_us-gaap_DisaggregationOfRevenueLineItems_1571" />
        <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Revenues" xlink:label="Locator_us-gaap_Revenues_1572" />
        <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="Locator_us-gaap_DisaggregationOfRevenueLineItems_1571" xlink:to="Locator_us-gaap_Revenues_1572" order="1.0" />
    </definitionLink>
</linkbase>
