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    <dei:EntityRegistrantName contextRef="P10_01_2025To10_01_2025" id="ixv-381">Fidus Investment Corporation</dei:EntityRegistrantName>
    <dei:EntityIncorporationStateCountryCode contextRef="P10_01_2025To10_01_2025" id="ixv-382">MD</dei:EntityIncorporationStateCountryCode>
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    <dei:EntityTaxIdentificationNumber contextRef="P10_01_2025To10_01_2025" id="ixv-384">27-5017321</dei:EntityTaxIdentificationNumber>
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    <dei:Security12bTitle contextRef="P10_01_2025To10_01_2025" id="ixv-396">Common Stock, par value $0.001 per share</dei:Security12bTitle>
    <dei:TradingSymbol contextRef="P10_01_2025To10_01_2025" id="ixv-397">FDUS</dei:TradingSymbol>
    <dei:SecurityExchangeName contextRef="P10_01_2025To10_01_2025" id="ixv-398">NASDAQ</dei:SecurityExchangeName>
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    <cef:LongTermDebtTableTextBlock contextRef="P10_01_2025To10_01_2025" id="ixv-180">&lt;div style="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman"&gt;On October&#160;1, 2025, Fidus Investment Corporation (the &#x201c;Company&#x201d;) entered into an underwriting agreement (the &#x201c;Underwriting Agreement&#x201d;) by and among the Company and Fidus Investment Advisors, LLC (the &#x201c;Adviser&#x201d;), on the one hand, and Raymond James&#160;&amp;amp; Associates, Inc. and ING Financial Markets LLC, as representatives of the several underwriters named in Exhibit A thereto, on the other hand, in connection with the issuance and sale of an additional $100.0&#160;million in aggregate principal amount of the Company&#x2019;s 6.750% Notes due 2030 (the &#x201c;New 2030 Notes&#x201d; and the issuance and sale thereof, the &#x201c;Offering&#x201d;). &lt;/div&gt;&lt;div style="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman"&gt;The Underwriting Agreement includes customary representations, warranties, and covenants by the Company and the Adviser. It also provides for customary indemnification by each of the Company, the Adviser, and the underwriters against certain liabilities and customary contribution provisions in respect of those liabilities. &lt;/div&gt;&lt;div style="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman"&gt;The foregoing description of the Underwriting Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of the Underwriting Agreement, which is filed as an exhibit hereto and incorporated by reference herein. &lt;/div&gt;</cef:LongTermDebtTableTextBlock>
    <cef:LongTermDebtPrincipal
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    <cef:LongTermDebtTitleTextBlock contextRef="P10_01_2025To10_01_2025" id="ixv-401">6.750% Notes due 2030</cef:LongTermDebtTitleTextBlock>
    <cef:LongTermDebtDividendsAndCovenantsTextBlock contextRef="P10_01_2025To10_01_2025" id="ixv-402">The Underwriting Agreement includes customary representations, warranties, and covenants by the Company and the Adviser. It also provides for customary indemnification by each of the Company, the Adviser, and the underwriters against certain liabilities and customary contribution provisions in respect of those liabilities.</cef:LongTermDebtDividendsAndCovenantsTextBlock>
    <cef:OutstandingSecuritiesTableTextBlock contextRef="P10_01_2025To10_01_2025" id="ixv-403">On October&#160;3, 2025, the Company caused notices to be issued to the holders of its 4.75% Notes due 2026 (CUSIP No.&#160;316500 AB3) (the &#x201c;2026 Notes&#x201d;) regarding the Company&#x2019;s exercise of its option to redeem, in full, $100.0&#160;million in aggregate principal amount of the issued and outstanding 2026 Notes, pursuant to Section&#160;1104 of the Base Indenture and Section&#160;1.01(h) of the Fourth Supplemental Indenture, dated as of December&#160;23, 2020, by and between the Company and the Trustee. The Company will redeem $100.0&#160;million in aggregate principal amount of the issued and outstanding 2026 Notes on November&#160;3, 2025 (the &#x201c;Redemption Date&#x201d;). The redemption price for the 2026 Notes equals 100% of the $100.0&#160;million aggregate principal amount of the 2026 Notes being redeemed on the Redemption Date, plus accrued and unpaid interest to, but excluding, the Redemption Date. A copy of the notice of redemption of the 2026 Notes is filed as Exhibit 99.1 hereto and incorporated by reference herein.</cef:OutstandingSecuritiesTableTextBlock>
    <cef:OutstandingSecurityTitleTextBlock contextRef="P10_01_2025To10_01_2025" id="ixv-404">4.75% Notes due 2026</cef:OutstandingSecurityTitleTextBlock>
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