Diamond Shamrock again turned down Pickens.

The Dallas-based oil company said its board unanimously rejected a $15-per-share tender offer for 20 million Diamond Shamrock common shares. The offer was made earlier this month by Lucky Partners, a partnership including an affiliate of Mesa Limited Partnership led by T. Boone Pickens Jr. and John M. Harbert III. The company called the offer “even more inadequate . . . than the earlier proposal” made by Mesa in December, a stock swap valued then at $2 billion. Diamond Shamrock said it will soon announce steps to “maximize” shareholders’ investments.