<PAGE>
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                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                         FORM 10-K/A (AMENDMENT NO. 1)

            /X/    Annual Report Pursuant to Section 13 or 15(d) of
                      the Securities Exchange Act of 1934
                           For the fiscal year ended
                                August 31, 1996
                                       or

          / /    Transition Report Pursuant to Section 13 or 15(d) of
                      the Securities Exchange Act of 1934
                            ------------------------

                                Commission File
                          Nos. 33-83868 and 333-11693
                            ------------------------

                         AMERICAN CRYSTAL SUGAR COMPANY
             (Exact name of registrant as specified in its charter)

<TABLE>
<S>                                  <C>
             MINNESOTA                            84-0004720
     (State of incorporation)          (I.R.S. Employer Identification
                                                   Number)

      101 NORTH THIRD STREET
        MOORHEAD, MN 56560                      (218) 236-4400
  (Address of principal executive      (Registrant's telephone number)
             offices)
</TABLE>

          SECURITIES REGISTERED PURSUANT TO SECTION 12(B) OF THE ACT:
                                      NONE
          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 requirements 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. / /
                            ------------------------
 
    As of November 22, 1996, 2,441 shares of the Registrant's Common Stock and
415,255 shares of the Registrant's Preferred Stock were outstanding. As there is
only a limited, private market for shares of the Registrant's stock and the
Registrant does not obtain information regarding the transfer price in
transactions between its members, the Registrant is not able to estimate the
aggregate market value of the Registrant's shares held by non-affiliates.
 
                      DOCUMENTS INCORPORATED BY REFERENCE
 
    Certain exhibits to this Report are incorporated by reference from the
Company's Registration Statement on Form S-1 (File number 33-83868), declared
effective on November 23, 1994, from the Company's Annual Report on Form 10-K
for the fiscal year ending August 31, 1995 and from the Company's Registration
Statement on Form S-1 (File number 333-11693), declared effective on November
13, 1996.
 
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--------------------------------------------------------------------------------


<PAGE>

                              REASON FOR AMENDMENT

    Amendment No. 1 to American Crystal Sugar Company Form 10K for the year 
ended August 31, 1996 is made to include a Financial Data Schedule that was 
inadvertently excluded from the original filing.

                                    PART IV.

ITEM 14.  EXHIBITS, FINANCIAL STATEMENT SCHEDULES AND REPORTS ON FORM 8-K
 
FINANCIAL STATEMENT SCHEDULES
 
    None
 
REPORTS ON FORM 8-K
 
    The Company was not required to and did not file any reports on Form 8-K
during the three months ended August 31, 1996.
 
EXHIBITS
 
<TABLE>
<S>        <C>
3(i)*      Restated Articles of Incorporation of American Crystal Sugar Company.
 
3(ii)***   Restated Bylaws of American Crystal Sugar Company.
 
5.1***     Opinion of Doherty, Rumble & Butler Professional Association.
 
10(f)***   Growers' Contract (5-year Agreement).
 
10(g)*     Growers' Contract (Annual Contract).
 
10(h)*     Coal Supply Agreement between Registrant and Spring Creek Coal Company, dated
             August 1, 1986.
 
10(i)*     Coal Transportation Agreement between Registrant and Northern Coal
             Transportation Company, dated August 1, 1986.
 
10(j)*     Beet Loading and Hauling Agreement between Registrant and Transystems, Inc.,
             dated May 18, 1993.
 
10(k)*     Form of Uniform Member Marketing Agreement between Registrant and United Sugars
             Corporation, dated January 1, 1994.
 
10(l)*     Trademark License Agreement between Registrant and United Sugars Corporation,
             dated November 1, 1993.
 
10(m)*     Uniform Member Marketing Agreement, Pool Basis between Registrant and Midwest
             Agri-Commodities Company, dated April 14, 1992.
 
10(n)*     Stipulation Agreement between Registrant and State of Minnesota Pollution
             Control Agency.
 
10(o)*     Master Agreement between Registrant, United Sugars Corporation, American
             Federation of Grain Millers, AFL-CIO, CLC, et al.
 
10(p)*     Loan Agreement between Registrant and St. Paul Bank for Cooperatives, dated
             December 20, 1993.
</TABLE>


                                       29
<PAGE>

<TABLE>
<S>        <C>
10(q)*     Amended and Restated Loan Agreement between Registrant and First Bank National
             Association, dated November 22, 1993.
 
10(r)*     Pension Contract and Amendments.
 
10(s)*     Compensation, Severance and Loan Agreement with Mr. J. Famalette, dated March 2,
             1992.
 
10(t)*     Compensation and Loan Agreement with Mr. J. Famalette, dated October 1, 1993.
 
10(u)*     Form of Operating Agreement between Registrant and ProGold Limited Liability
             Company.
 
10(v)*     Form of Member Control Agreement between Registrant and ProGold Limited
             Liability Company.
 
10(w)*     Administrative Services Agreement between Registrant and ProGold Limited
             Liability Company.
 
10(x)*     Uniform Member Marketing Agreement.
 
10(y)**    Coal Supply Agreement between Registrant and Spring Creek Coal Company, dated
             August 25, 1995 (Confidential Treatment Requested as to certain provisions).
 
10(z)**    Coal Transportation Agreement between Registrant and Northern Coal
             Transportation Company, dated August 25, 1995 (Confidential Treatment
             Requested as to certain provisions).
 
10(aa)***  Gas Sales Contract between Registrant and Coastal Gas Marketing Company, dated
             as of March 20, 1996 (Confidential treatment requested as to certain
             provisions).
 
10(bb)***  Employment Agreement with Mr. Daniel McCarty, dated as of April 8, 1996.
 
10(cc)***  Form of Subscription Agreement for Preferred Stock (Current Members).
             Subscription Document A.
 
10(dd)***  Form of Indication of Interest in Purchasing Additional Shares of Preferred
             Stock (Current Members). Subscription Document B.
 
10(ee)***  Form of Indication of Interest in Purchasing Shares of Preferred Stock
             (Prospective Members). Subscription Document C.
 
10(ff)***  Form of Subscription Agreement (Prospective Members). Subscription Document D.
 
10(gg)***  Form of Representations and Warranties. Subscription Document E.

23 #       Consent of Independent Public Accountant.

27.1       Financial Data Schedule.

99 #       Consents of Directors-Elect.
</TABLE>

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

  * Incorporated by reference from the Company's Registration Statement on Form
    S-1 (File No. 33-83868), declared effective November 23, 1994.
 
 ** Incorporated by reference from the Company's Annual Report on Form 10-K for
    the fiscal year ending August 30, 1995.
 
*** Incorporated by reference from the Company's Registration Statement on Form
    S-1 (File number 333-11693), declared effective on November 13, 1996.

#   Previously filed.



                                       30
<PAGE>
                                   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.
 
                                AMERICAN CRYSTAL SUGAR COMPANY
 
                                By             /s/ DANIEL J. MCCARTY
                                     -----------------------------------------
                                                 Daniel J. McCarty,
                                              CHIEF EXECUTIVE OFFICER

Dated: January 8, 1997

    Pursuant to the requirements of the Securities Act of 1933, this report has
been signed by the following persons in the capacities and on the dates
indicated.


<TABLE>
<CAPTION>
          SIGNATURE                       TITLE                    DATE
------------------------------  --------------------------  -------------------
<C>                             <S>                         <C>
    /s/ DANIEL J. MCCARTY
------------------------------  Chief Executive Officer      January 8, 1997
      Daniel J. McCarty
 
     /s/ JAMES J. HORVATH
------------------------------  Chief Financial Officer      January 8, 1997
       James J. Horvath
 
     /s/ SAMUEL S.M. WAI
------------------------------  Corporate Controller         January 8, 1997
       Samuel S.M. Wai
 
     /s/  MICHAEL ASTRUP
------------------------------  Director                     January 8, 1997
       Michael Astrup

     /s/  JERRY BITKER
------------------------------  Director                     January 8, 1997
       Jerry Bitker

       /s/ PAUL BORGEN
------------------------------  Director                     January 8, 1997
         Paul Borgen

     /s/ AIME J. DUFAULT
------------------------------  Director                     January 8, 1997
       Aime J. Dufault
 
    /s/ STEVEN M. GOODWIN
------------------------------  Director                     January 8, 1997
      Steven M. Goodwin
</TABLE>

                                       31
<PAGE>

<TABLE>
<CAPTION>
          SIGNATURE                       TITLE                    DATE
------------------------------  --------------------------  -------------------
<C>                             <S>                         <C>
     /s/ COURT G. HANSON
------------------------------  Director                     January 8, 1997
       Court G. Hanson
 
       /s/ LONN M. KIEL
------------------------------  Director                     January 8, 1997
         Lonn M. Kiel
 
      /s/ DAVID KRAGNES
------------------------------  Director                     January 8, 1997
        David Kragnes
 
   /s/ FRANCIS KRITZBERGER
------------------------------  Director                     January 8, 1997
     Francis Kritzberger
 
       /s/ WAYNE LANGEN
------------------------------  Director                     January 8, 1997
         Wayne Langen

     /s/ PATRICK D. MAHAR
------------------------------  Director                     January 8, 1997
       Patrick D. Mahar
 
      /s/ BARRY W. MALME
------------------------------  Director                     January 8, 1997
        Barry W. Malme

    /s/ RONALD REITMEIER
------------------------------  Director                     January 8, 1997
      Ronald Reitmeier

    /s/ G. TERRY STADSTAD
------------------------------  Director                     January 8, 1997
      G. Terry Stadstad
 
     /s/ ROBERT VIVATSON
------------------------------  Director                     January 8, 1997
       Robert Vivatson
</TABLE>


                                      32