Nearly as quickly as it ousted its former chief executive, Merrill Lynch has found a new C.E.O.

John Thain, the chief executive of N.Y.S.E. Euronext, will become chief executive and chairman of Merrill.

Thain "brings unparalleled leadership experience and knowledge of the complexities of global capital markets as well as the skills required to operate a large financial services company," Alberto Cribiore, Merrill's interim chairman, said in a statement.

Thain, 52, will start on December 1.

His appointment, which  was first reported this afternoon by the New York Post, comes less than two weeks after Stan O'Neal departed amid an embarrassing $8.4 billion write-down on investments tied to subprime mortgages.

Many challenges await Thain, the first outsider to lead Merrill in the firm's history. He must restore investors' confidence, improve employee morale, and grapple with an Securities and Exchange Commission investigation into how the firm disclosed its write-downs. And some analysts expect the firm to take further write-downs on its nearly $21 billion in collateralized debt obligations and mortgages.

Thain, in an interview with CNBC this afternoon, said that he was excited about the opportunities at Merrill, even though Wall Street in general has not "seen a bottom" on write-downs.  

Noting that he had run Goldman Sachs' mortgage business in the 1980's, Thain said, "taking risk is fine; you just need to size it appropriately."

From the beginning, Thain had been one of the leading candidates to pick up the reins at Merrill. But he was not seen as the front-runner: Many on Wall Street expected Laurence Fink, the chief executive of the invesment management firm BlackRock, in which Merrill has nearly a 50 percent stake, to be appointed C.E.O. But Fink was never offered the job.

Two top Merrill executives, Robert McCann and Greg Fleming, had also been seen in the running.

A former top executive at Goldman Sachs, Thain is considered to be an expert on technology systems and an astute manager of operations. Some called him the ideal candidate to overhaul the stumbling behemoth Citigroup, whose chief executive, Charles Prince, left just days after O'Neal. In the interview with CNBC, Thain declined to comment on whether he had held job talks with Citi. He did say he had been talking to Merrill's board for the last two weeks.

Since taking over the N.Y.S.E. in January 2004, Thain has transformed the the world's largest stock market. He inherited an institution still reeling from the scandal and ill will over the ouster of Richard Grasso as its chief executive. Thain led the exchange through its evolution from a clubby, not-for-profit organization to a publicly traded corporation. He engineered deals with Archipelago and with Euronext, which have made the New York exchange a more electronic, global platform for trading stocks and other products.

Thain shares some qualities with his predecessor, the New York Times' DealBook blog notes. Both are known for their dispassionate manner, in contrast to the more gregarious, back-slapping Wall Street chieftains. And they both hold M.B.A.'s from Harvard.

The arrival of Thain will underscore the immense influence alumni of Goldman Sachs have on the nation's financial system.

Robert Rubin, a former co-president of Goldman and former Treasury Secretary, is the new chairman of Citigroup and is the leading the search for its new chief executive. Thain's successor at the N.Y.S.E., Duncan Niederauer, is also a former Goldman executive. And Treasury Secretary Henry Paulson is a former C.E.O. of Goldman.