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                                                                    Exhibit 10.1



                                    FOURTH AMENDMENT dated as of July 15, 1999
                           (this "AMENDMENT"), to the Credit Agreement, dated as
                           of September 29, 1998 (as amended, supplemented or
                           otherwise modified from time to time, the "CREDIT
                           AGREEMENT"), among PSINET INC., a corporation
                           organized under the laws of the State of New York
                           (the "BORROWER"), the several banks and other
                           financial institutions and entities from time to time
                           parties thereto (the "LENDERS"), THE CHASE MANHATTAN
                           BANK, as administrative agent (the "ADMINISTRATIVE
                           AGENT") for the Lenders, Fleet National Bank, as
                           syndication agent for the Lenders, and The Bank of
                           New York as documentation agent for the Lenders.


                  WHEREAS, pursuant to the Credit Agreement, the Lenders have
agreed to make certain loans to the Borrower; and

                  WHEREAS the Borrower has requested that certain provisions of
the Credit Agreement be modified in the manner provided for in this Amendment,
and the Lenders are willing to agree to such modifications as provided for in
this Amendment.


                  NOW, THEREFORE, the parties hereto hereby agree as follows:

                  1. DEFINED TERMS. Capitalized terms used and not defined
herein shall have the meanings given to them in the Credit Agreement, as amended
hereby.

                  2.  AMENDMENTS TO THE CREDIT AGREEMENT.

                  Section 6.01 of the Credit Agreement is hereby amended by
deleting clause (ix) thereof in its entirety and substituting the following
therefor:

             (ix) Indebtedness permitted by Section 10.08(b)(iii) of the Senior
         Note Indenture, in an aggregate principal amount not to exceed the
         lesser of (x) the product of (A) two and (B) the net cash proceeds
         received by the Borrower since the date of such indenture from sales of
         its capital stock in


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         public offerings or to strategic investors and (y) $1,600,000,000 less
         the aggregate principal amount of the 1998 High Yield Notes.

                  3. NO OTHER AMENDMENTS; CONFIRMATION. Except as expressly
amended, waived, modified and supplemented hereby, the provisions of the Credit
Agreement are and shall remain in full force and effect.

                  4. REPRESENTATIONS AND WARRANTIES. The Borrower hereby
represents and warrants to the Administrative Agent and the Lenders as of the
date hereof as follows:

                  (a) No Default or Event of Default has occurred and is
continuing.

                  (b) The execution, delivery and performance by the Borrower of
this Amendment have been duly authorized by all necessary corporate and other
action and do not and will not require any registration with, consent or
approval of, notice to or action by, any person (including any governmental
agency) in order to be effective and enforceable. The Credit Agreement as
amended by this Amendment constitutes the legal, valid and binding obligation of
the Borrower, enforceable against it in accordance with its terms, subject only
to the operation of the Bankruptcy Code and other similar statutes for the
benefit of debtors generally and to the application of general equitable
principles.

                  (c) All representations and warranties of the Borrower
contained in the Credit Agreement (other than representations or warranties
expressly made only on and as of the Effective Date) are true and correct as of
the date hereof.

                  5. EFFECTIVENESS. This Amendment shall become effective only
upon the satisfaction in full of the following conditions precedent: The
Administrative Agent shall have received counterparts hereof, duly executed and
delivered by the Borrower and the Required Lenders; and

                  6. EXPENSES. The Borrower agrees to reimburse the
Administrative Agent for its out-of-pocket expenses in connection with this
Amendment, including the reasonable fees, charges and disbursements of Cravath,
Swaine & Moore, counsel for the Administrative Agent.

                  7. GOVERNING LAW; COUNTERPARTS. (a) This Amendment and the
rights and obligations of the parties hereto shall be governed by, and construed
and interpreted in accordance with, the laws of the State of New York.



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                  (b) This Amendment may be executed by one or more of the
parties to this Amendment on any number of separate counterparts, and all of
said counterparts taken together shall be deemed to constitute one and the same
instrument. This Amendment may be delivered by facsimile transmission of the
relevant signature pages hereof.


                  IN WITNESS WHEREOF, the parties hereto have caused this
Amendment to be duly executed and delivered by their respective proper and duly
authorized officers as of the day and year first above written.


                              PSINET INC.

                                    by
                                          /s/ D.N. Kunkel
                                          -----------------------------------
                                          Name:  D.N. Kunkel
                                          Title: Executive Vice President


                              THE CHASE MANHATTAN BANK,
                              individually and as Administrative Agent,

                                    by
                                          /s/ John J. Huber
                                          -----------------------------------
                                          Name:  John J. Huber III
                                          Title: Managing Director


                              FLEET NATIONAL BANK,
                              individually and as Syndication Agent,

                                    by
                                          /s/ Andrew C. Wigren
                                          -----------------------------------
                                          Name:  Andrew C. Wigren
                                          Title: Assistant Vice President


                              THE BANK OF NEW YORK,
                              individually and as Documentation Agent,

                                    by
                                          /s/ Gerry Granovsky
                                          -----------------------------------

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                                          Name:  Gerry Granovsky
                                          Title: Vice President


                              BANKBOSTON, N.A.,

                                    by
                                          /s/ Cindy Chen
                                          -----------------------------------
                                          Name:  Cindy Chen
                                          Title: Director