Authorities are investigating why eight billion osyters have died in farms on the Atlantic coast in the past few days.
Researchers at the science body Ifremer believe a new bacteria or virus is behind the deaths. Other theories under investigation include a strain of poisonous seaweed or algae.
Between 40-100% of one-year-old oysters are dying depending on the areas.
Fisheries Minister Michel Barnier has announced aid to the farms, who could lose millions.
Mr Barnier said the deaths were one of the worst catastrophes to hit the industry since 1970 when a virus decimated the species.
He said a body would be formed to look into individual cases and financial aid, while the procedures for an agricultural catastrophe would also be put in place.
France produces 130,000 tonnes of oysters each year. The current batch was destined for consumption over Christmas 2009.
Oysters are a popular Christmas dish in France which produces 130,000 tonnes annually.