MDU Resources Acquires Ames Sand & Gravel, Inc.

BISMARCK, N.D. – July 5, 2007 – MDU Resources Group, Inc. (NYSE:MDU) announced today that it has acquired Ames Sand & Gravel, Inc., a ready-mix concrete company headquartered in Fargo, N.D. Ames will operate as part of Knife River Corporation, the construction materials and mining subsidiary of MDU Resources. Financial details of the acquisition were not disclosed. MDU Resources anticipates the acquisition will be accretive to 2007 earnings per share. “We are pleased to announce the acquisition of Ames,” said Terry D. Hildestad, president and chief executive officer of MDU Resources. “Ames has a proud history as a successful, well- managed company. Additionally, as a materials-based company, it is a good strategic fit for our corporation as we continue to focus on growing our core businesses – construction materials, energy and utility resources.” On average, Ames produces and pours approximately 90,000 cubic yards of ready-mixed concrete per year. Its 2006 revenues were $7.8 million. It employs approximately 30 people during peak season and operates a fleet of 23 ready-mix trucks and one concrete pump truck. The company completed the construction of a new state-of-the-industry batch plant in the spring of 2007. “We are excited to bring Ames into Knife River,” said William Schneider, president and chief executive officer of Knife River. “This acquisition allows us to begin operating in one of the strongest and fastest-growing markets in eastern North Dakota and western Minnesota. It bridges the gap between our existing operations in Bismarck, N.D., and St. Cloud, Minn.” Ames will be operated as part of Atlas, Inc., a Knife River subsidiary headquartered in Bismarck. Jeff Reinholz will continue to manage the day-to-day operations of the company. The information in this release includes forward-looking statements, including statements by the president and chief executive officer of MDU Resources and the president and chief executive officer of Knife River, as well as the statement with respect to the anticipated effect of the transaction upon earnings per share, within the meaning of Section 21E of the Securities Exchange Act of 1934. Although the company believes that its expectations are based on reasonable assumptions, actual results may differ materially. Important factors that could cause actual results to differ materially from those in the forward-looking statements include the ability to effectively integrate the operations. For a discussion of other important factors that could cause actual results to differ, refer to Item 1A – Risk Factors in MDU Resources’ most recent Form 10-K and Form 10-Q.