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            <identifier scheme="http://www.sec.gov/CIK">0001513363</identifier>
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        <period>
            <startDate>2026-10-20</startDate>
            <endDate>2026-10-20</endDate>
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        <measure>iso4217:USD</measure>
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    <dei:AmendmentFlag contextRef="P10_20_2026To10_20_2026" id="ixv-328">false</dei:AmendmentFlag>
    <dei:EntityCentralIndexKey contextRef="P10_20_2026To10_20_2026" id="ixv-329">0001513363</dei:EntityCentralIndexKey>
    <dei:DocumentType contextRef="P10_20_2026To10_20_2026" id="ixv-341">8-K</dei:DocumentType>
    <dei:DocumentPeriodEndDate contextRef="P10_20_2026To10_20_2026" id="ixv-342">2026-10-20</dei:DocumentPeriodEndDate>
    <dei:EntityRegistrantName contextRef="P10_20_2026To10_20_2026" id="ixv-343">Fidus Investment Corporation</dei:EntityRegistrantName>
    <dei:EntityIncorporationStateCountryCode contextRef="P10_20_2026To10_20_2026" id="ixv-344">MD</dei:EntityIncorporationStateCountryCode>
    <dei:EntityFileNumber contextRef="P10_20_2026To10_20_2026" id="ixv-345">814-00861</dei:EntityFileNumber>
    <dei:EntityTaxIdentificationNumber contextRef="P10_20_2026To10_20_2026" id="ixv-346">27-5017321</dei:EntityTaxIdentificationNumber>
    <dei:EntityAddressAddressLine1 contextRef="P10_20_2026To10_20_2026" id="ixv-347">1603 Orrington Avenue</dei:EntityAddressAddressLine1>
    <dei:EntityAddressAddressLine2 contextRef="P10_20_2026To10_20_2026" id="ixv-348">Suite 1005</dei:EntityAddressAddressLine2>
    <dei:EntityAddressCityOrTown contextRef="P10_20_2026To10_20_2026" id="ixv-349">Evanston</dei:EntityAddressCityOrTown>
    <dei:EntityAddressStateOrProvince contextRef="P10_20_2026To10_20_2026" id="ixv-350">IL</dei:EntityAddressStateOrProvince>
    <dei:EntityAddressPostalZipCode contextRef="P10_20_2026To10_20_2026" id="ixv-351">60201</dei:EntityAddressPostalZipCode>
    <dei:CityAreaCode contextRef="P10_20_2026To10_20_2026" id="ixv-352">847</dei:CityAreaCode>
    <dei:LocalPhoneNumber contextRef="P10_20_2026To10_20_2026" id="ixv-353">859-3940</dei:LocalPhoneNumber>
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    <dei:Security12bTitle contextRef="P10_20_2026To10_20_2026" id="ixv-358">Common Stock, par value $0.001 per share</dei:Security12bTitle>
    <dei:TradingSymbol contextRef="P10_20_2026To10_20_2026" id="ixv-359">FDUS</dei:TradingSymbol>
    <dei:SecurityExchangeName contextRef="P10_20_2026To10_20_2026" id="ixv-360">NASDAQ</dei:SecurityExchangeName>
    <dei:EntityEmergingGrowthCompany contextRef="P10_20_2026To10_20_2026" id="ixv-361">false</dei:EntityEmergingGrowthCompany>
    <cef:LongTermDebtTableTextBlock contextRef="P10_20_2026To10_20_2026" id="ixv-187">&lt;div style="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman"&gt;As previously disclosed, on May&#160;29, 2026, Fidus Investment Corporation (the &#x201c;Company&#x201d;) issued $120.0 in aggregate principal amount of its 6.625% Notes due 2029 (the &#x201c;Restricted Notes&#x201d;) in a transaction not requiring registration under the Securities Act of 1933, as amended (the &#x201c;Securities Act&#x201d;). The Restricted Notes were originally issued pursuant to a Note Purchase Agreement, dated May&#160;29, 2026, by and between the Company and the initial purchasers of the Restricted Notes (the &#x201c;Private Offering&#x201d;). &lt;/div&gt;&lt;div style="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman"&gt;In connection with the Private Offering, the Company entered into a registration rights agreement with the initial purchasers of the Restricted Notes (the &#x201c;Registration Rights Agreement&#x201d;), pursuant to which the Company agreed to offer to exchange the Restricted Notes for a like principal amount of its 6.625% Notes due 2029 registered under the Securities Act (the &#x201c;Exchange Notes&#x201d;) in order to satisfy its obligations under the Registration Rights Agreement (the &#x201c;Exchange Offer&#x201d;). The Exchange Notes were registered under the Securities Act pursuant to the Company&#x2019;s registration statement on Form &lt;span style="white-space: nowrap; letter-spacing: 0px; top: 0px;display:inline;"&gt;N-14&lt;/span&gt; (File &lt;span style="white-space: nowrap; letter-spacing: 0px; top: 0px;display:inline;"&gt;No.&#160;333-297156)&lt;/span&gt; previously filed with the Securities and Exchange Commission (the &#x201c;SEC&#x201d;) and were issued under the Indenture (as defined below) and a final prospectus dated September&#160;16, 2026. The Exchange Offer closed on October&#160;20, 2026. Following such closing, holders of the initial Restricted Notes now hold the same aggregate principal amount of Exchange Notes. &lt;/div&gt;</cef:LongTermDebtTableTextBlock>
    <cef:LongTermDebtPrincipal
      contextRef="P10_20_2026To10_20_2026"
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      id="ixv-362"
      unitRef="Unit_USD">120</cef:LongTermDebtPrincipal>
    <cef:LongTermDebtTitleTextBlock contextRef="P10_20_2026To10_20_2026" id="ixv-363">6.625% Notes due 2029</cef:LongTermDebtTitleTextBlock>
    <cef:LongTermDebtDividendsAndCovenantsTextBlock contextRef="P10_20_2026To10_20_2026" id="ixv-203">&lt;div style="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman"&gt;The Indenture contains certain affirmative and negative covenants, including covenants requiring the Company to comply with the asset coverage ratio required by the Investment Company Act of 1940, as amended (the &#x201c;1940 Act&#x201d;). These covenants are subject to important limitations and exceptions that are set forth in the Indenture. &lt;/div&gt;</cef:LongTermDebtDividendsAndCovenantsTextBlock>
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