PROSPECTUS SUPPLEMENT
                       TO PROSPECTUS DATED MARCH 7, 2000

                                     [LOGO]

                            USINTERNETWORKING, INC.

                                  $125,000,000
             7% CONVERTIBLE SUBORDINATED NOTES DUE NOVEMBER 1, 2004
                 AND 5,030,181 SHARES OF COMMON STOCK ISSUABLE
                          UPON CONVERSION OF THE NOTES

                         88,915 SHARES OF COMMON STOCK
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    This Prospectus Supplement supplements and amends the Prospectus dated
March 7, 2000 relating to the 7% Convertible Subordinated Notes due November 1,
2004, the shares of common stock, par value $.001 per share, issuable upon
conversion of the Notes and the offering by five of our stockholders of up to
88,915 shares of our common stock acquired in private placement transactions.

    The table on pages 86, 87 and 88 of the Prospectus, which sets forth
information with respect to the Selling Security Holders and the respective
amounts of Notes beneficially owned by each Selling Security Holder that may be
offered under the Prospectus (as supplemented and amended), is amended so that
the following new line item is added to the table of Selling Security Holders:


                                                                             
                                                                           *
Greek Catholic Union II.........................       15,000                               905


    The Prospectus, together with this Prospectus Supplement, constitutes the
prospectus required to be delivered by Section 5(b) of the Securities Act of
1933, as amended, with respect to offers and sales of the Notes, our common
stock issuable upon conversion of the Notes and the shares of common stock held
by the five stockholders identified in the Prospectus. All references in the
Prospectus to "this Prospectus" are hereby amended to read "this Prospectus (as
supplemented and amended)."

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    INVESTING IN THE NOTES OR OUR COMMON STOCK INVOLVES SUBSTANTIAL RISKS. SEE
"RISK FACTORS" BEGINNING ON PAGE 9 OF THE PROSPECTUS.

    NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES
COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES OR PASSED UPON THE
ADEQUACY OR ACCURACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.

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           The date of this Prospectus Supplement is March 27, 2000.