Omaha, Neb. ? Billionaire investor Warren Buffett assured stockholders in his annual letter Saturday he knew who would replace him as chairman of Berkshire Hathaway Inc. if “I should die tonight.”

While the 75-year-old said “I feel terrific,” and that he plans to remain at the helm, Buffett announced the board had unanimously agreed on a successor. He’s just not saying who.

“We have three managers at Berkshire who are reasonably young and fully capable of being CEO,” he said.

In his eagerly awaited letter, Buffett said his board was prepared to oust him should the need arise “from my decay, particularly if this decay is accompanied by my delusionally thinking that I am reaching new peaks of managerial brilliance.”

Buffett built a 1956 partnership of four relatives and three close friends into a holding company with total assets of $198.3 billion at the end of 2005. Berkshire owns furniture, carpet, jewelry and candy companies, restaurants, natural gas and corporate jet firms and has major investments in such companies as Coca-Cola, Anheuser-Busch and Wells Fargo & Co.

The question of Buffett’s successor has long been a topic of speculation, but Buffett has never dropped any names. Buffett repeated Saturday that his current duties would be split into two parts, with one executive handling investments and another dealing with day-to-day operations.