The largest non-government investor in Cnooc Ltd. has sold all of its shares because of concern over the Chinese oil firm's efforts to acquire Unocal Corp.

William Blair & Co. sold its stock in Cnooc for a total of about $160 million in recent weeks, said David Merjan, a portfolio manager at the Chicago-based investment firm.

The firm, which held the stock on behalf of mutual funds and pension funds, concluded that Cnooc was offering too much money for Unocal, a California-based energy company, and that the bid indicated a change in approach for a company that previously shied from acquisitions.

"They thought they would be able to continue down the path of organic growth and that acquisitions wouldn't be necessary," Merjan said. "This showed a change of course."

Merjan said that until the effort to purchase Unocal, Cnooc had made decisions in the best interest of shareholders. But he said the effort to buy Unocal appears to be based on government policy, not business reasons.

"We think that this is . . . just part of a strategy supported by the government to essentially acquire as much in the way of energy assets as they possibly can," Merjan said.

Cnooc officials have said they made the decision in the best interests of the company, not to further government policy. China National Offshore Oil Corp., which is state-controlled, owns 70 percent of Cnooc.

Merjan said that to acquire Unocal, Cnooc probably would have to increase its bid beyond what he already considered an inflated price. Cnooc has offered $18.5 billion for Unocal.

Chevron Corp. made an earlier offer for Unocal valued at about $16.5 billion. Chevron officials have said its offer is more appealing because the deal has been approved by regulators. Cnooc has not yet been granted regulatory approval to acquire Unocal.

Unocal shareholders are scheduled to vote Aug. 10 on whether to accept the Chevron bid.

William Blair was Cnooc's top non-government shareholder as of March 31, when it held slightly more than 0.5 percent of Cnooc shares. Bloomberg News reported yesterday that William Blair had sold its stock.

Spokesmen for several other large investors could not or would not say whether they had sold Cnooc stock.