Results of Operations · Reg FD Disclosure

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K/A (Amendment No. 1) CURRENT REPORT PURSUANT TO SECTION 13 OR 15 (d) OF THE SECURITIES EXCHANGE ACT OF 1934 Date of Report (Date of earliest event reported) May 4, 2015 MDU Resources Group, Inc. (Exact name of registrant as specified in its charter) Delaware 1-3480 41-0423660 (State or other jurisdiction of (Commission File Number) (I.R.S. Employer Identification No.)   incorporation)     1200 West Century Avenue P.O. Box 5650 Bismarck, North Dakota 58506-5650 (Address of principal executive offices) (Zip Code) Registrant’s telephone number, including area code (701) 530-1000 Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:   o Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)   o Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)   o Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))   o Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))   Item 2.02. Results of Operations and Financial Condition and Item 7.01. Regulation FD Disclosure. On May 5, 2015, MDU Resources Group, Inc. (the “Company”) filed a Current Report on Form 8-K (the “Form 8-K”) reporting first quarter 2015 earnings.  When the Form 8-K was filed by EDGAR with the U. S. Securities and Exchange Commission it was inadvertently labeled as an Exhibit 99 Earnings Release, and the Exhibit 99 Earnings Release was mislabeled as the Form 8-K.  The Company is filing this amendment to the Form 8-K to correct the EDGAR filing labels.   On May 4, 2015, the Company issued a press release announcing first quarter 2015 earnings.  A copy of the press release, which the Company is furnishing to the Securities and Exchange Commission, is attached as Exhibit 99 and incorporated by reference herein.      Item 9.01. Financial Statements and Exhibits. (d)   Exhibits               99 Press release issued May 4, 2015, regarding first quarter 2015 earnings. 2 SIGNATURE 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 hereunto duly authorized.   MDU RESOURCES GROUP, INC.     Date   May 6, 2015 By /s/ Nathan W. Ring   Nathan W. Ring   Vice President, Controller and   Chief Accounting Officer 3 EXHIBIT INDEX Exhibit Number Description of Exhibit     99 Press release issued May 4, 2015, regarding first quarter 2015 earnings.      4