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UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON , D.C. 20549 FORM 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31 , 2020 OR TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from Large accelerated filer Non - accelerated filer to Commission file number 1-03480 MDU RESOURCES GROUP INC ( Exact name of registrant as specified in its charter ) Delaware ( State or other jurisdiction of incorporation or organization ) Securities registered pursuant to Section 12 ( b ) of the Act : Title of each class Common Stock , par value $ 1.00 per share 1200 West Century Avenue P.O. Box 5650 Bismarck , North Dakota 58506-5650 ( Address of principal executive offices ) ( Zip Code ) 30-1133956 ( I.R.S. Employer Identification No. ) ( 701 ) 530-1000 ( Registrant's telephone number , including area code ) Trading symbol ( s ) MDU Name of each exchange on which registered New York Stock Exchange Securities registered pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes > No. Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . Yes □ No > . 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 No □ . Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( § 232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes > No. Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or an emerging growth company . See the definitions of " large accelerated filer , " " accelerated filer , " " smaller reporting company , " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Accelerated filer Smaller reporting company Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . < Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Act ) . Yes □ No > . State the aggregate market value of the voting common stock held by non - affiliates of the registrant as of June 30 , 2020 : $ 4,447,584,104 . Indicate the number of shares outstanding of the registrant's common stock , as of February 11 , 2021 : 200,522,277 shares . DOCUMENTS INCORPORATED BY REFERENCE Relevant portions of the registrant's 2021 Proxy Statement , to be filed no later than 120 days from December 31 , 2020 , are incorporated by reference in Part III , Items 10 , 11 , 12 , 13 and 14 of this Report .