Unocal declined comment on Mesa’s latest move.

A spokesman for Los Angeles-based Unocal, parent of Union Oil Co. of California, said the company would not make a statement on the disclosure that Mesa Partners has increased its stake in the firm to 14.77 million shares, or 8.5%. Mesa Partners is headed by T. Boone Pickens, chairman of Mesa Petroleum of Amarillo, Tex. The partnership has said it intends to raise its Unocal stake to about 15%. In trading on the New York Stock Exchange, the price of Unocal rose $1.25 to close at $46.875.