PARIS  A large French mutual bank, Groupe Caisse d’Épargne, said Friday that it had lost 600 million euros, or $807 million, as a result of unauthorized derivatives trading by a team on the bank’s own account.

The lender, which is in merger talks with a French rival, said in a statement that the loss was the result of “extreme market volatility” last week, and that the position had been closed.

The losses resulted from bad bets on derivatives linked to the direction of the CAC-40, the French equivalent of the Dow Jones industrial average, according to French officials.

A spokeswoman for the bank sought to distinguish these losses from the multibillion-dollar loss inflicted on Société Générale by Jérôme Kerviel, the rogue trader who made headlines early this year. “The traders took more risk in terms of the volume and the amount that was authorized,” she said.