NEW YORK — Merrill Lynch & Co. on Thursday recorded its biggest quarterly loss since being founded 94 years ago, after the world's largest brokerage took almost $15 billion worth of write-downs from bad subprime mortgage bets.

The fourth-quarter loss was $9.91 billion as Merrill Lynch became the latest Wall Street bank bloodied by the ongoing credit crisis. Its massive write-offs came largely from the shrinking value of securities backed by mortgages, which have soured as borrowers are unable to repay them on time.

Merrill Lynch joins rival Wall Street investment houses Morgan Stanley and Bear Stearns Cos. in posting losses in the last three months of fiscal 2007. Citigroup Inc., the nation's largest bank, reported on Tuesday a quarterly loss of almost $10 billion, the largest in its 196-year history.

John Thain, the new chief executive at Merrill Lynch, said he believes this will be the bulk of the company's write-downs from its subprime mortgage exposure. But he would not speculate about what 2008 might hold in store in other areas.

The huge housing-driven shortfalls come as weak economic data have intensified fears of a recession and have increased pressure on the government for an economic stimulus plan. There is also increasing evidence that mortgage delinquencies are spilling over into credit cards, home equity loans and other areas.