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                                                                   Exhibit 10.13


                  SUPPLEMENT NO. 1 dated as of October 14, 1999, to the
                  Indemnity, Subrogation and Contribution Agreement dated as of
                  June 23, 1999 (as the same may be amended, supplemented or
                  otherwise modified from time to time, the "INDEMNITY,
                  SUBROGATION AND CONTRIBUTION AGREEMENT"), among ANTEON
                  CORPORATION, a Virginia corporation (the "BORROWER") each
                  Subsidiary of the Borrower listed on Schedule I thereto (the
                  GUARANTORS"), and MELLON BANK, N.A., an national banking
                  association, ("MELLON"), as collateral agent (the "COLLATERAL
                  AGENT") for the Secured Parties (as defined in the Credit
                  Agreement referred to below).

     A. Reference is made to (a) the Credit Agreement dated as of June 23, 1999
(as amended, supplemented or otherwise modified from time to time, the "CREDIT
AGREEMENT"), among the Borrower, the lenders from time to time party thereto
(the "LENDERS"), Credit Suisse First Boston, as administrative agent for the
Lenders (in such capacity, the "ADMINISTRATIVE AGENT"), and as issuing bank,
Mellon, as Collateral Agent, swingline lender and syndication agent, and
Deutsche Bank A. G., as documentation agent, and (b) the Subsidiary Guarantee
Agreement dated as of June 23, 1999, among the Guarantors and the Collateral
Agent (the "GUARANTEE AGREEMENT").

     B. Capitalized terms used herein and not otherwise defined herein shall
have the meanings assigned to such terms in the Indemnity, Subrogation and
Contribution Agreement and the Credit Agreement.

     C. The Borrower and the Guarantors have entered into the Indemnity,
Subrogation and Contribution Agreement in order to induce the Lenders to make
Loans and the Issuing Bank to issue Letters of Credit. Pursuant to Section 5.12
of the Credit Agreement, each Domestic Subsidiary of the Borrower (other than
any Inactive Subsidiary) that was not in existence or not such a Subsidiary on
the date of the Credit Agreement is required to enter into the Guarantee
Agreement as a Guarantor upon becoming a Domestic Subsidiary (or ceasing to be
an Inactive Subsidiary). Section 12 of the Indemnity, Subrogation and
Contribution Agreement provides that additional Subsidiaries of the Borrower
may become Guarantors under the Indemnity, Subrogation and Contribution
Agreement by  execution and delivery of an instrument in the form of this
Supplement. The undersigned Subsidiary of the Borrower (the "NEW GUARANTOR") is
executing this Supplement in accordance with the requirements of the Credit
Agreement to become a Guarantor under the Indemnity, Subrogation and
Contribution Agreement in order to induce the Lenders to make additional Loans
and the

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Issuing Bank to issue additional Letters of Credit and as consideration for
Loans previously made and Letters of Credit previously issued.

     Accordingly, the Collateral Agent and the New Guarantor agree as follows:

     SECTION 1. In accordance with Section 12 of the Indemnity, Subrogation
and Contribution Agreement, the New Guarantor by its signature below becomes
a Guarantor under the Indemnity, Subrogation and Contribution Agreement with
the same force and effect as if originally named therein as a Guarantor and
the New Guarantor hereby agrees to all the terms and provisions of the
Indemnity, Subrogation and Contribution Agreement applicable to it as a
Guarantor thereunder. Each reference to a "Guarantor" in the Indemnity,
Subrogation and Contribution Agreement shall be deemed to include the New
Guarantor. The Indemnity, Subrogation and Contribution Agreement is hereby
incorporated herein by reference.

     SECTION 2. The New Guarantor represents and warrants to the Collateral
Agent and the other Secured Parties that this Supplement has been duly
authorized, executed and delivered by it and constitutes its legal, valid and
binding obligation, enforceable against it in accordance with its terms.

     SECTION 3. This Supplement may be executed in counterparts (and by
different parties hereto on different counterparts), each of which shall
constitute an original, but all of which when taken together shall constitute a
single contract. This Supplement shall become effective when the Collateral
Agent shall have received counterparts of this Supplement that, when taken
together, bear the signatures of the New Guarantor and the Collateral Agent.
Delivery of an executed signature page to this Supplement by fax transmission
shall be as effective as delivery of a manually signed counterpart of this
Supplement.

     SECTION 4. Except as expressly supplemented hereby, the Indemnity,
Subrogation and Contribution Agreement shall remain in full force and effect.

     SECTION 5. THIS SUPPLEMENT SHALL BE GOVERNED BY, AND CONSTRUED IN
ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

     SECTION 6. In case any one or more of the provisions contained in this
Supplement should be held invalid, illegal or unenforceable in any respect,
neither party hereto shall be required to comply with such provision for so
long as such provision is held to be invalid, illegal or unenforceable, but the
validity, legality and enforceability of the remaining provisions contained
herein and in the Indemnity, Subrogation and Contribution Agreement shall not
in any way be affected or impaired. The parties hereto shall endeavor in
good-faith negotiations


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to replace the invalid, illegal or unenforceable provisions with valid
provisions the economic effect of which comes as close as possible to that of
the invalid, illegal or unenforceable provisions.

     SECTION 7. All communications and notices hereunder shall be in writing
and given as provided in Section 7 of the Indemnity, Subrogation and
Contribution Agreement. All communications and notices hereunder to the New
Guarantor shall be given to it at the address set forth under its signature.

     SECTION 8. The New Guarantor agrees to reimburse the Collateral Agent
for its reasonable out-of-pocket expenses in connection with this Supplement,
including the reasonable fees, other charges and disbursements of counsel for
the Collateral Agent.

     IN WITNESS WHEREOF, the New Guarantor and the Collateral Agent have duly
executed this Supplement to the Indemnity, Subrogation and Contribution
Agreement as of the day and year first above written.

                                       ANTEON-CITI, LLC,

                                       by: /s/ Carlton B. Crenshaw
                                          -----------------------------------
                                          Name: Carlton B. Crenshaw
                                          Title: Vice President
                                          Address: 3211 Jermantown Rd.
                                                   Fairfax, VA 22030

                                       MELLON BANK, N.A., as Collateral Agent,

                                       by: /s/ Leslie A. Grizzard
                                          -----------------------------------
                                          Name: Leslie A. Grizzard
                                          Title: Vice President