<PAGE>   1

                                   FORM 10K/A
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                       -----------------------------------

                       AMENDMENT TO APPLICATION OR REPORT
                  FILED PURSUANT TO SECTION 12, 13 OR 15(D) OF
                       THE SECURITIES EXCHANGE ACT OF 1934

Commission File number 1-10216:

                          CHIEFTAIN INTERNATIONAL, INC.

               (Exact name of registrant as specified in charter)

           ALBERTA, CANADA                                  NONE
--------------------------------------     -----------------------------------
   (State or other jurisdiction of         (I.R.S. Employer Identification No.)
    incorporation or organization)


  1201 TD Tower, 10088 - 102 Avenue,
       Edmonton, Alberta, Canada                         T5J  2Z1
--------------------------------------     -----------------------------------

Registrants telephone number, including area code:   (780) 425-1950

                                 AMENDMENT NO. 1


The undersigned registrant hereby amends the following items, financial
statements, exhibits or other portions of its annual report on Form 10K for the
year ended December 31, 2000 as set forth in the page attached hereto:

     PART II, ITEM 5. MARKET FOR THE REGISTRANT'S SECURITIES AND RELATED
     STOCKHOLDER MATTERS
     Price History for March 1 to March 14, 2001 is corrected to read American
     Stock Exchange (US Dollars) High 29.65, Low 22.00 and Toronto Stock
     Exchange (Cdn. Dollars) High 45.50, Low 34.20

Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this amendment to be signed on its behalf by the
undersigned, thereunto duly authorized.


                                           CHIEFTAIN INTERNATIONAL, INC.
                                           -----------------------------
                                           (Registrant)




                                           /s/ S. A. Milner
                                           -------------------------------------
                                           S. A. Milner
                                           President and Chief Executive Officer
                                           and Principal Financial Officer


Date: May 4, 2001
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                                     PART II


ITEM 5.   MARKET FOR THE REGISTRANT'S SECURITIES AND RELATED STOCKHOLDER MATTERS

The principal United States market in which our Common Shares are traded is the
American Stock Exchange. The Common Shares are also traded on the Toronto Stock
Exchange. The high and low prices of our Common Shares (the "Common Shares")
during each quarter since December 31, 1998 are shown below.

<TABLE>
<CAPTION>
                            Price History of Chieftain International, Inc. Common Shares
                            ------------------------------------------------------------
                             American Stock Exchange            Toronto Stock Exchange
                                    (US dollars)                    (Cdn. dollars)
                            --------------------------         -------------------------
                               High             Low              High             Low
                            ----------       ---------         ---------        --------
<S>                          <C>             <C>               <C>              <C>
1999
   First quarter             $   15.50       $    9.56         $   24.00        $  14.50
   Second quarter                18.63           12.25             26.95           19.25
   Third quarter                 22.75           17.44             34.00           25.90
   Fourth quarter                20.38           14.06             30.25           21.00
2000
   First quarter                 20.38           13.38             27.85           19.50
   Second quarter                22.25           17.63             33.20           25.45
   Third quarter                 22.50           15.88             33.50           23.60
   Fourth quarter                27.75           19.31             41.90           29.20


2001
   January                       27.75           22.25             41.50           33.45
   February                      25.16           22.02             38.90           34.00
   March 1 to March 14           29.65           22.00             45.50           34.20
</TABLE>

The Common Shares were held by 116 Shareholders of record on December 31, 2000.
We estimate that investment dealers and other nominees hold Common Shares for
approximately 2,200 beneficial holders.

At the present time it is not our policy to declare regular dividends on the
Common Shares. This policy is under periodic review by the Board of Directors
and is subject to change at any time depending on our earnings and our financial
requirements. Dividends may be paid on the Common Shares provided that all
dividends on the preferred shares of Chieftain International Funding Corp. have
been paid. All dividends on the preferred shares have been paid.