<PAGE>
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549


                                   FORM 10-K/A


[X]      ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
         EXCHANGE ACT OF 1934

         FOR THE FISCAL YEAR ENDED DECEMBER 31, 2002.

[ ]      TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
         EXCHANGE ACT OF 1934

       FOR THE TRANSITION PERIOD FROM __________TO ___________.

                         COMMISSION FILE NUMBER: 0-29370

                              ULTRA PETROLEUM CORP.
             (Exact Name of Registrant as Specified in Its Charter)

<TABLE>
<S>                                                <C>
             YUKON TERRITORY, CANADA                              N/A
 (Jurisdiction of Incorporation or Organization)   (I.R.S. Employer Identification No.)

363 NORTH SAM HOUSTON PARKWAY EAST, SUITE 1200
          HOUSTON, TEXAS                                         77060
(Address of Principal Executive Offices)                       (Zip Code)
</TABLE>

                                  281-876-0120
              (Registrant's Telephone Number, Including Area Code)

SECURITIES REGISTERED PURSUANT TO SECTION 12(b) OF THE ACT:

<TABLE>
<CAPTION>
                                                  NAME OF EACH EXCHANGE
     TITLE OF EACH CLASS                           ON WHICH REGISTERED
     -------------------                           -------------------
<S>                                              <C>
        Common Shares                            American Stock Exchange
      without par value                           Toronto Stock Exchange
</TABLE>

SECURITIES REGISTERED PURSUANT TO SECTION 12(g) OF THE ACT: NONE

Indicate by check mark whether the registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during
the preceding 12 months (or for such shorter period that the registrant was
required to file such reports), and (2) has been subject to such filing
requirement for the past 90 days. YES [X] NO [ ]

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405
of Regulation S-K is not contained herein, and will not be contained, to the
best of registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. [ ]

Indicate by check mark whether the registrant is an accelerated filer (as
defined in Exchange Act Rule 12b-2). YES [ ] NO [ X ]

As of March 3, 2003, the Registrant had 74,087,668 common shares outstanding,
and the aggregate market value of the common shares held by non-affiliates was
approximately $659,380,245 based upon the closing price of $ 8.90 per share for
the common stock on March 3, 2003, as reported on the American Stock Exchange.

Documents incorporated by reference: The definitive Proxy Statement for the 2003
Annual Meeting of Stockholders, which will be filed with the Securities and
Exchange Commission within 120 days after December 31, 2002, is incorporated by
reference in Part III of this Form 10-K.


                                       1
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ITEM 15. EXHIBITS, FINANCIAL STATEMENT SCHEDULES, AND REPORTS ON FORM 8-K

      This Form 10-K/A is being filed for the sole purpose of filing
Exhibit 10.1 to the Company's Form 10-K which was originally filed on
March 28, 2003. The exhibit was omitted in error from the previously
filed Form 10-K by the Company's filing agent.

Exhibit Number    Description
--------------    -----------

      3.1         Articles of Incorporation of Ultra Petroleum Corp. -
                  (incorporated by reference to Exhibit 3.1 of the Company's
                  Quarterly Report on Form 10-Q for the period ended June 30,
                  2001)


                                      2
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<TABLE>
<S>               <C>
         3.2      By-Laws of Ultra Petroleum Corp. - (incorporated by reference
                  to Exhibit 3.2 of the Company's Quarterly Report on Form 10-Q
                  for the period ended June 30, 2001)

         4.1      Specimen Common Share Certificate - (incorporated by reference
                  to Exhibit 4.1 of the Company's Quarterly Report on Form 10-Q
                  for the period ended June 30, 2001)

         10.1     First Amendment to First Amended and Restated Credit Agreement
                  dated November 4, 2002 among Ultra Resources, Inc., Bank One
                  N.A., Union Bank of California, N.A., Hibernia National Bank,
                  Guaranty Bank, FSB and Compass Bank

         10.2     First Amended and Restated Credit Agreement dated March 1,
                  2002 among Bank One, N.A., Union Bank of California, N.A.,
                  Guaranty Bank, FSB, Hibernia National Bank, Ultra Resources,
                  Inc. and Banc One Capital Markets, Inc. (incorporated by
                  reference to Exhibit 10.1 to the Company's Annual Report on
                  Form 10-K for the period ended December 31, 2001)

         10.3     First Amendment to Credit Agreement dated July 19, 2001
                  (incorporated by reference to Exhibit 10.1 to the Company's
                  Quarterly Report on Form 10-Q for the period ended September
                  30, 2001)

         10.4     Credit Agreement dated March 22, 2000 (incorporated by
                  reference to Exhibit 10.1 of the Company's Quarterly Report on
                  Form 10-Q for the period ended June 30, 2001)

         10.5     Ratification of and Amendment to Mortgage dated February 15,
                  2001 (incorporated by reference to Exhibit 10.2 of the
                  Company's Quarterly Report on Form 10-Q for the period ended
                  June 30, 2001)

         10.6     Articles of Merger dated July 16, 2001 (incorporated by
                  reference to Exhibit 10.1 to the Company's Quarterly Report on
                  Form 10-Q for the period ended September 30, 2001)

         10.7     Plan of Merger and Reorganization dated July 16, 2001
                  (incorporated by reference to Exhibit 10.1 to the Company's
                  Quarterly Report on Form 10-Q for the period ended September
                  30, 2001)

         21.1     Subsidiaries of the Company (incorporated by reference to
                  Exhibit 21.1 to the Company's Annual Report on Form 10-K for
                  the period ended December 31, 2001)

         23.1     Consent of Netherland, Sewell & Associates, Inc.

         99.1     Certification of Chief Executive Officer

         99.2     Certification of Chief Financial Officer
</TABLE>

                                      3
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                                   SIGNATURES

         Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, the Registrant has duly caused this report to be signed on
its behalf by the undersigned, thereunto duly authorized.

                              ULTRA PETROLEUM CORP.


Date: March 25, 2003          By:  /s/ Michael D. Watford
                                   Name:   Michael D. Watford
                                   Title:  Chairman of the Board,
                                           Chief Executive Officer and President

         Pursuant to the requirements of the Securities Exchange Act of 1934,
this report has been signed below by the following persons on behalf of the
Registrant and in the capacities and on the dates indicated.

<TABLE>
<CAPTION>
             SIGNATURE                                TITLE                                     DATE
<S>                                       <C>                                           <C>
     /s/ Michael D. Watford               Chairman of the Board,
     ----------------------               Chief Executive Officer and President         March 25, 2003
     Michael D. Watford

     /s/ W. Charles Helton
     ----------------------
     W. Charles Helton                    Director                                      March 25, 2003

     /s/ James E. Nielson
     ----------------------
     James E. Nielson                     Director                                      March 25, 2003

     /s/ Robert E. Rigney
     ----------------------
     Robert E. Rigney                     Director                                      March 25, 2003

     /s/ James C. Roe
     ----------------------
     James C. Roe                         Director                                      March 25, 2003

     /s/ F. Fox Benton III
     ----------------------
     F. Fox Benton III                    Chief Financial Officer                       March 25, 2003
</TABLE>


                                      4
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                                 EXHIBIT INDEX

Exhibit Number    Description
--------------    -----------
         3.1      Articles of Incorporation of Ultra Petroleum Corp. -
                  (incorporated by reference to Exhibit 3.1 of the Company's
                  Quarterly Report on Form 10-Q for the period ended June 30,
                  2001)

         3.2      By-Laws of Ultra Petroleum Corp. - (incorporated by reference
                  to Exhibit 3.2 of the Company's Quarterly Report on Form 10-Q
                  for the period ended June 30, 2001)

         4.1      Specimen Common Share Certificate - (incorporated by reference
                  to Exhibit 4.1 of the Company's Quarterly Report on Form 10-Q
                  for the period ended June 30, 2001)

         10.1     First Amendment to First Amended and Restated Credit Agreement
                  dated November 4, 2002 among Ultra Resources, Inc., Bank One
                  N.A., Union Bank of California, N.A., Hibernia National Bank,
                  Guaranty Bank, FSB and Compass Bank

         10.2     First Amended and Restated Credit Agreement dated March 1,
                  2002 among Bank One, N.A., Union Bank of California, N.A.,
                  Guaranty Bank, FSB, Hibernia National Bank, Ultra Resources,
                  Inc. and Banc One Capital Markets, Inc. (incorporated by
                  reference to Exhibit 10.1 to the Company's Annual Report on
                  Form 10-K for the period ended December 31, 2001)

         10.3     First Amendment to Credit Agreement dated July 19, 2001
                  (incorporated by reference to Exhibit 10.1 to the Company's
                  Quarterly Report on Form 10-Q for the period ended September
                  30, 2001)

         10.4     Credit Agreement dated March 22, 2000 (incorporated by
                  reference to Exhibit 10.1 of the Company's Quarterly Report on
                  Form 10-Q for the period ended June 30, 2001)

         10.5     Ratification of and Amendment to Mortgage dated February 15,
                  2001 (incorporated by reference to Exhibit 10.2 of the
                  Company's Quarterly Report on Form 10-Q for the period ended
                  June 30, 2001)

         10.6     Articles of Merger dated July 16, 2001 (incorporated by
                  reference to Exhibit 10.1 to the Company's Quarterly Report on
                  Form 10-Q for the period ended September 30, 2001)

         10.7     Plan of Merger and Reorganization dated July 16, 2001
                  (incorporated by reference to Exhibit 10.1 to the Company's
                  Quarterly Report on Form 10-Q for the period ended September
                  30, 2001)

         21.1     Subsidiaries of the Company (incorporated by reference to
                  Exhibit 21.1 to the Company's Annual Report on Form 10-K for
                  the period ended December 31, 2001)

         23.1     Consent of Netherland, Sewell & Associates, Inc.

         99.1     Certification of Chief Executive Officer

         99.2     Certification of Chief Financial Officer