The chairman of French savings bank Caisse d'Epargne has quit over the loss of 600m euros (£466m) in a "trading incident" amid global market chaos.
Charles Milhaud said he accepted full responsibility for the lost cash and is expected to leave without a pay-off.
The bank's director general and chief financial officer have resigned from their posts.
The loss was made public soon after the bank had set out plans to merge with another mutual bank, Banque Populaire.
In a statement reported by the AFP news agency, Mr Milhaud said: "This loss is the result of the exceptional volatility of the markets during this period and the breach of instructions that the board and myself gave.
"I accept entire responsibility, however. Having devoted my entire life to the Caisse d'Epargne, I have never at any moment considered trying to avoid my responsibilities," he added.
"Those who know me know that I am not a man of money. I am not asking for any indemnity."
Merger on course
French President Nicolas Sarkozy has said Caisse d'Epargne's losses were "unacceptable".
And Finance Minister Christine Lagarde called for a special audit of all French banks.
Ministers hope the resignations will restore confidence in the banking sector and avoid the need for another expensive bailout.
Caisse d'Epargne bet that stock markets would rise, just as the credit crunch sent shares tumbling, says the BBC's Duncan Bartlett.
It said the problems would not derail its planned merger with Banque Populaire, which would make it one of France's largest banks with 480bn euros in deposits.
The two banks together are the majority shareholders in the investment bank Natixis, which has been among the worst-hit in France by the US sub-prime mortgage crisis.
The latest loss at Caisse d'Epargne comes after France's Societe Generale saw a loss of 4.9bn euros earlier this year, in the wake of a trading incident which it blamed on trader Jerome Kerviel.
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