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                                                                     EXHIBIT 4.1


                          FIRST SUPPLEMENTAL INDENTURE

         FIRST SUPPLEMENTAL INDENTURE (this "Supplemental Indenture"), dated as
of June 15, 2000, by and between METAMOR WORLDWIDE, INC., a Delaware corporation
(the "Company"), PSINET INC., a New York corporation ("PSINet"), and THE BANK OF
NEW YORK, as trustee (the "Trustee").

                                    RECITALS

         WHEREAS, the Company (formerly named Corestaff, Inc., a Delaware
corporation), as issuer, and the Trustee are parties to that certain Indenture,
dated as of August 15, 1997 (the "Indenture"), providing for the issuance of the
Company's 2.94% Convertible Subordinated Notes due 2004 (the "Notes"); and

         WHEREAS, there are now outstanding under the Indenture, Notes in the
aggregate principal amount at maturity of $227,000,000; and

         WHEREAS, pursuant to the terms of an Agreement and Plan of Merger dated
as of March 21, 2000 between the Company, PSINet and PSINet Shelf IV Inc.,
PSINet Shelf IV Inc. has merged with and into the Company, with the Company
surviving as a wholly-owned subsidiary of PSINet (the "Merger"); and

         WHEREAS, in connection with the Merger, each outstanding share of
Common Stock (as defined in the Indenture) has been converted into the right to
receive 0.9 of a share of PSINet's common stock; and

         WHEREAS, in accordance with Sections 15.6 and 3.7(e) of the Indenture,
the Company and PSINet wish to enter this Supplemental Indenture to provide
that, among other things, the Notes will hereafter be convertible into PSINet's
common stock in lieu of Common Stock of the Company, and that the provisions of
Section 3.7 of the Indenture relating to redemption obligations of the Company
in the event of a Fundamental Change (as defined in the Indenture) shall apply
with respect to transactions involving PSINet's common stock rather than Common
Stock of the Company; and

         WHEREAS, Section 11.1 of the Indenture provides that Company, PSINet
and the Trustee may enter into an indenture or indentures supplemental to the
Indenture without the consent of the holders of Notes for the purpose of adding
any provisions with respect to the conversion rights of the holders of Notes
pursuant to the requirements of Section 15.6 of the Indenture and the redemption
obligations of the Company pursuant to the requirements of Section 3.7(e) of the
Indenture; and

         WHEREAS, the Company and PSINet desire and have requested the Trustee
to join with them in entering into this Supplemental Indenture for the purpose
of amending the Indenture in certain respects as permitted by said Section 11.1
of the Indenture; and


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         WHEREAS, (1) the Company has delivered to the Trustee an Officers'
Certificate and Opinion of Counsel relating to this Supplemental Indenture as
contemplated by Section 11.5 of the Indenture, (2) PSINet has delivered to the
Trustee an Opinion of Counsel, and (3) each of the Company and PSINet has
satisfied all other conditions required under Article XI of the Indenture to
enable the Company, PSINet and the Trustee to enter into this Supplemental
Indenture.

         NOW, THEREFORE, in consideration of the foregoing and for other good
and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the Company and PSINet covenant and agree with the Trustee for the
equal and proportionate benefit of the respective holders from time to time of
the Notes as follows:

                                    ARTICLE I
                             AMENDMENTS TO INDENTURE

         1.1 AMENDMENTS RELATING TO SECTION 3.7 OF THE INDENTURE. Pursuant to
Section 3.7(e) of the Indenture, the Indenture is hereby amended as follows:

                  (a) As used in the definitions of the terms "Applicable Price"
         and "Fundamental Change" in the Indenture, the defined term "Common
         Stock" shall, subject to any further adjustments pursuant to Section
         15.6 of the Indenture, refer to the common stock, par value $.01 per
         share, of PSINet or shares of any class or classes resulting from any
         reclassification or reclassifications thereof and which have no
         preference in respect of dividends or of amounts payable in the event
         of any voluntary or involuntary liquidation, dissolution or winding up
         of PSINet and which are not subject to redemption by PSINet; provided,
         that if at any time there shall be more than one such resulting class,
         the shares of each such class then so issuable shall be substantially
         in the proportion which the total number of shares of such class
         resulting from all such reclassifications bears to the total number of
         shares of all such classes resulting from all such reclassifications
         ("PSINet Common Stock").

                  (b) The term "Reference Market Price" is amended to mean
         initially $21.3889, and in the event of any adjustment to the
         Conversion Rate pursuant to Section 15.5 after the date hereof, the
         Reference Market Price shall also be adjusted so that the Reference
         Market Price after giving effect to any such adjustment shall equal the
         Reference Market Price immediately prior to such adjustment multiplied
         by a fraction, the numerator of which is the Conversion Rate
         immediately prior to such adjustment and the denominator of which is
         the Conversion Rate after such adjustment.

         1.2 AMENDMENTS RELATING TO ARTICLE XV OF THE INDENTURE. From and after
the date hereof, pursuant to Section 15.6 of the Indenture, PSINet agrees that
the Notes shall be convertible into PSINet Common Stock (as defined in Section
1.1(a) hereof) as provided in Article XV of the Indenture, after giving effect
to the changes thereto effected by this Supplemental Indenture. To effect the
foregoing, the Indenture is hereby amended as follows:

                  (a) All references in Article XV of the Indenture to "Common
         Stock" shall be deemed to refer to PSINet Common Stock as defined in
         this Supplemental Indenture.



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                  (b) The third paragraph of Section 15.2 of the Indenture is
         hereby amended to read in its entirety as follows:

                           As promptly as practicable after satisfaction of the
                           requirements for conversion set forth above, subject
                           to compliance with any restrictions on transfer if
                           shares issuable on conversion are to be issued in a
                           name other than that of the Noteholder (as if such
                           transfer were a transfer of the Note or Notes (or
                           portion thereof) so converted), (x) PSINet shall
                           issue and deliver to the Company, and the Company
                           shall deliver to such holder at the office or agency
                           maintained by the Company for such purpose pursuant
                           to Section 5.2, a certificate or certificates for the
                           number of full shares of PSINet Common Stock issuable
                           upon such conversion of such Note or portion thereof
                           in accordance with the provisions of this Article,
                           and (y) the Company shall deliver to such holder at
                           the office or agency maintained by the Company for
                           such purpose pursuant to Section 5.2 a check or cash
                           in respect of any fractional interest in respect of a
                           share of PSINet Common Stock arising upon such
                           conversion, as provided in Section 15.3. In case any
                           Note of a denomination greater than $1,000 shall be
                           surrendered for partial conversion, and subject to
                           Section 2.3, the Company shall execute and the
                           Trustee shall authenticate and deliver to the holder
                           of the Note so surrendered, without charge to him, a
                           new Note or Notes in authorized denominations in an
                           aggregate principal amount equal to the unconverted
                           portion (including Original Issue Discount) of the
                           surrendered Note.

                  (c) The reference to "the Company" in the fourth paragraph of
         Section 15.2 of the Indenture shall be deemed to refer instead to
         PSINet.

                  (d) Section 15.4 of the Indenture is hereby amended to read in
         its entirety as follows:

                           As of the date hereof, the Conversion Rate shall be
                           21.36573 shares of PSINet Common Stock per $1,000
                           principal amount at maturity of Notes converted. Such
                           Conversion Rate shall be subject to such further
                           adjustments from time to time as are provided in
                           Section 15.5 of the Indenture as amended by this
                           Supplemental Indenture.

                  (e) Each reference in Section 15.5 to "the Company" shall be
         deemed to refer instead to PSINet, and references to the "Board of
         Directors" in such Section 15.5 shall be deemed to refer to the board
         of directors of PSINet or any committee thereof duly authorized to act
         on its behalf, except that:



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                           (i) the reference to "the Company" in the language
                  "best interests of the Company" in Section 15.5(i) shall be
                  deemed to refer instead to the Company or PSINet;

                           (ii) the last two references to "the Company" in
                  Section 15.5(i) shall continue to refer to the Company instead
                  of PSINet;

                           (iii) the second reference to "the Company" in
                  Section 15.5(j) shall continue to refer to the Company instead
                  of PSINet; and

                           (iv) Section 15.5(k) shall be amended to read in its
                  entirety as follows:

                           In any case in which this Section 15.5 provides that
                           an adjustment shall become effective immediately
                           after a record date for an event, (i) PSINet may
                           defer until the occurrence of such event issuing and
                           delivering to the Company the additional shares of
                           PSINet Common Stock issuable upon such conversion by
                           reason of the adjustment required by such event over
                           and above the PSINet Common Stock issuable upon such
                           conversion before giving effect to such adjustment,
                           and (ii) the Company may defer until the occurrence
                           of such event delivering to the holder of any Note
                           converted after such record date and before the
                           occurrence of such event such additional shares of
                           PSINet Common Stock or paying to such holder any
                           amount in cash or additional shares in lieu of any
                           fractional share pursuant to Section 15.3.

                  (f) The first paragraph of Section 15.6 of the Indenture shall
         apply to PSINet as if all references therein to "the Company" instead
         referred to PSINet.

                  (g) All references in Sections 15.7, 15.8 and 15.10 of the
         Indenture to "the Company" shall be deemed to refer instead to PSINet,
         except that the last reference to "the Company" in Section 15.10 shall
         continue to refer to the Company instead of PSINet.

                  (h) PSINet hereby appoints the Trustee to act as conversion
         agent for purposes of complying with its obligations under the
         Indenture as amended by this Supplemental Indenture.

         1.3 FORM OF NOTE. The form of Note attached as Exhibit A to the
Indenture is hereby deemed modified as appropriate to reflect the amendments
reflected by this Supplemental Indenture. Any Notes authenticated and delivered
after the close of business on the date of this Supplemental Indenture in
substitution for Notes then outstanding and all Notes presented or delivered to
the Trustee on and after that date for such purpose shall be stamped, imprinted
or otherwise legended by the Trustee, with a notation as follows:

                  "Effective as of June 15, 2000, the Company, the Trustee and
         PSINet Inc., a New York corporation ("PSINet"), entered into a First
         Supplemental Indenture providing that, among



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         other things, the Notes are thereafter convertible into PSINet's common
         stock in lieu of Common Stock of the Company, and that the provisions
         of Section 3.7 of the Indenture relating to redemption obligations of
         the Company in the event of a Fundamental Change apply with respect to
         transactions involving PSINet's common stock rather than Common Stock
         of the Company. Reference is hereby made to such First Supplemental
         Indenture, copies of which are on file with the Trustee, for a
         description of the amendments made therein."

                                   ARTICLE II
                                  MISCELLANEOUS

         2.1 DEFINITIONS. Capitalized terms used herein without definition shall
have the meanings assigned to them in the Indenture. For all purposes of this
Supplemental Indenture, except as otherwise herein expressly provided or unless
the context otherwise requires, the words "herein," "hereof" and "hereby" and
other words of similar import used in this Supplemental Indenture refer to this
Supplemental Indenture as a whole and not to any particular section hereof.

         2.2 RATIFICATION OF INDENTURE; SUPPLEMENTAL INDENTURE PART OF
INDENTURE. Except as expressly amended hereby, the Indenture is in all respects
ratified and confirmed and all the terms, conditions and provisions thereof
shall remain in full force and effect. This Supplemental Indenture shall form a
part of the Indenture for all purposes, and every holder of Notes heretofore or
hereafter authenticated and delivered under the Indenture shall be bound hereby
and all terms and conditions of both shall be read together as though they
constitute a single instrument, except that in the case of conflict the
provisions of this Supplemental Indenture shall control.

         2.3 NO ASSUMPTION BY PSINET. Except as expressly provided in this
Supplemental Indenture, nothing herein shall be interpreted as an assumption by
PSINet of any obligations of the Company under the Indenture or the Notes,
including any obligation of the Company to make payment of principal, premium or
interest on the Notes.

         2.4 GOVERNING LAW. This Supplemental Indenture shall be governed by and
construed in accordance with the laws of the State of New York without regard to
its conflicts of laws principles.

         2.5 TRUSTEE MAKES NO REPRESENTATION. The Trustee makes no
representation as to the validity or sufficiency of this Supplemental Indenture.

         2.6 COUNTERPARTS. The parties may sign any number of copies or
counterparts of this Supplemental Indenture. Each signed copy shall be an
original, but all of them together represent the same agreement.

         2.7 EFFECT OF HEADINGS. The Section headings herein are for convenience
only and shall not affect the construction thereof.

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         IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed as of the date first above written.

                                        METAMOR WORLDWIDE, INC.


                                        By:
                                            ------------------------------------
                                        Name:  Edward L. Pierce
                                        Title: Executive Vice President
                                                 & Chief Financial Officer


                                        PSINET INC.


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------


                                        THE BANK OF NEW YORK, as Trustee


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------



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