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                                                                  Exhibit 10.1.2

                  FIRST AMENDMENT TO STOCK PURCHASE AGREEMENT

          THIS FIRST AMENDMENT TO STOCK PURCHASE AGREEMENT (this "Amendment") is
made as of this 4th day of February 2000 by and between PaeTec Corp., a Delaware
corporation (the "Company"), PaeTec Communications, Inc., a Delaware corporation
(the "Subsidiary"), and Arunas A. Chesonis (the "Shareholder").

                                   RECITALS
                                   --------

          A.  The Company, the Subsidiary and the Shareholder are parties to a
Stock Purchase Agreement dated as November 16, 1998 (the "Stock Purchase
Agreement").

          B.  The Board of Directors of the Company has authorized the issuance
and sale (the "Series A Preferred Stock Placement") of 134,000 shares of a new
series of preferred stock of the Company, designated the Series A Convertible
Preferred Stock, to the purchasers (the "Purchasers") listed on the Schedule of
Purchasers to, and pursuant to the terms and conditions of, an Equity Purchase
Agreement (the "Purchase Agreement").

          C.  As a condition to the consummation of the Series A Preferred Stock
Placement, the Purchasers have required that the Company, the Subsidiary the
Shareholder amend the Stock Purchase Agreement to clarify that, to the extent
that any securities are required to be excluded from a registration pursuant to
the "cut-back" provisions of the piggyback registration rights granted to the
Purchaser pursuant to the Stock Purchase Agreement, the securities to be
included in such registration shall be determined on a pro rata basis among the
                                                       --- ----
holders of shares participating in the offering pursuant to registration rights
granted by the Company, based on the number of shares of common stock requested
to be included by each such holder in such registration.

          D.  The parties hereto desire to amend the Stock Purchase Agreement to
induce the Purchasers to consummate the Series A Preferred Stock Placement.


                                   AGREEMENT
                                   ---------

          1.  Defined Terms. All capitalized terms used in this Amendment
              -------------
without definition shall have the meanings given to such terms in the Stock
Purchase Agreement.
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          2.  Amendment of Section 3(b).  The fourth and fifth sentences of
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Section 3(b) of the Stock Purchase Agreement are hereby deleted and replaced and
superseded in their entirety with the following sentence:

     "Notwithstanding any other provision of this Section 3, if the managing
     underwriter advises in writing the Company and the Shareholder that
     marketing factors require a limitation of the number of shares of common
     stock to be underwritten and sold in such offering, the managing
     underwriter may exclude some or all of the shares of common stock to be
     sold in such offering from such registration, and the shares to be included
     in such registration shall be allocated pro rata among the holders of
                                             --- ----
     shares participating in the offering pursuant to registration rights
     granted by the Company (including demand and piggyback registration
     rights), based on the number of shares of common stock requested to be
     included by each holder in such registration."

All other terms and conditions of the Stock Purchase Agreement remain in full
force and effect.

          3.  Binding Effect.  This Amendment shall be binding upon and inure to
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the benefit of the parties hereto and their heirs, executors, administrators,
successors and assigns.

          4.  Governing Law.  This Amendment shall be governed by, and construed
              -------------
and enforced in accordance with, the laws of the State of New York, except that
if any provision of this Amendment or any part of any such provision would be
illegal, invalid or enforceable under such laws in connection with a suit or
proceeding validly instituted in another jurisdiction, then the laws of such
other jurisdiction shall govern insofar as is necessary to sustain the legality,
validity or enforceability of such provision or any part of such provision.

          5.  Captions.  Captions to the Sections in this Amendment are for the
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convenience of the parties only and shall not affect the meaning or
interpretation of this Amendment.

          6.  Enforceability and Interpretation.  It is the intention of the
              ---------------------------------
parties to this Amendment that the terms and provisions contained in this
Amendment shall be enforceable to the fullest extent permitted by law.  If any
term or provision of this Amendment or the application thereof to any Person or
circumstance is construed to be illegal, invalid or unenforceable, in whole or
in part, then such term or provision shall be construed in such a manner as to
permit its enforceability under applicable law to the fullest extent permitted
by such law.  In

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any case, the remaining terms and provisions of this Amendment or the
application thereof to any Person or circumstance, except those terms and
provisions which have been held illegal, invalid or unenforceable, shall remain
in full force and effect.

          7.  Counterparts.  This Amendment may be executed in one or more
              ------------
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

          8.  Additional Documents.  Each party hereto agrees to execute any and
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all documents, instruments, certificates and communications deemed to be
necessary or advisable by the Company to effectuate the purposes of this
Amendment.

                           [signature page follows]

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          IN WITNESS WHEREOF, the parties hereto have executed and delivered
this Amendment with full force and effect as of the day and year first written
above.

                                   PAETEC CORP.


                                   By:   /s/ Arunas A. Chesonis
                                        -------------------------------
                                   Its:    CEO, Chairman and President
                                        -------------------------------


                                   PAETEC COMMUNICATIONS, INC.

                                   By:   /s/ Arunas A. Chesonis
                                        -------------------------------
                                   Its:    CEO, Chairman and President
                                        -------------------------------


                                       /s/ Arunas A. Chesonis
                                   ------------------------------------
                                   Arunas A. Chesonis

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