PARIS — The French bank Société Générale said Wednesday that it would write off another 1.4 billion euros in troubled American mortgage assets in the fourth quarter of 2009, but that it would still manage “a slight profit.”

Explaining the write-off, equivalent to about $2 billion, the bank said its analysis of American housing data had led it to a “much stricter assessment” of the valuation assumptions of certain United States mortgage securities.

“Over all, the group expects to post a slight profit in terms of estimated net income,” the bank said in a statement. Results for 2009 would show “generally satisfactory operating performances,” including “strong performances in corporate and investment banking,” it said.

Shares of Société Générale, second in France only to BNP Paribas in terms of market value, fell 2.9 percent in Paris on Wednesday. They had gained about 40 percent last year.

The write-downs resulted largely from deterioration in the loans underlying collateralized debt obligations, as well as a revaluing of credit-default swaps and other financial liabilities.

In the third quarter, Société Générale recorded 500 million euros of write-downs on its credit portfolio hedges and financial liabilities. The International Monetary Fund estimated in October that the credit crisis would ultimately bring total write-downs at banks and other financial institutions of about $3.4 trillion.

Results for the quarter will be helped by a capital gain of 600 million euros that the bank is receiving from the merger of its asset management business with that of its French banking rival Crédit Agricole. The two businesses were combined under the name of Amundi.

Société Générale strengthened its business in the fourth quarter by purchasing from the French-Belgian lender Dexia the remaining 20 percent stake in Crédit du Nord, and by raising 4.8 billion euros in equity in the stock market in October.