Oyster mortality was monitored at multiple sites along the French coastline (around 5 500 km) between 1993 and 2005. Mortality data for one- and two-year-old oysters were collected over 10–12 years in 39 oyster culture sites bordering 3 different "seas": the English Channel, Bay of Biscaye and Mediterranean. Combining these data with records from environmental monitoring databases, 11 of these sites had consistent chronological data sets including both environmental data and mortality records. Mortality in one-year-old oysters was clearly summer mortality (49% of their annual mortality) whereas mortality of two-year-olds occurred mostly in spring (51%). Analysis of variance revealed that "coastal area" was the main influence on mortality of one-year-olds (77.5%% of the variance) and that "year studied" was the main influence on mortality of two-year-olds (60.6% of the variance). The highest mortalities occurred in Marennes and in several sites in Brittany for both age groups, and in Veys Bay (Normandy) for two-year-old oysters only. Environmental parameters were then analysed to investigate which of these might influence summer mortality. Principal Component Analyses revealed that environmental factors such as chlorophyll a (food resource indicator) and salinity (watershed effect) influence oyster mortality. Chlorophyll a concentration (10% of the variance), water temperature (7% of the variance) and turbidity (5% of the variance) are the main significant factors for the mortality of one-year-olds, while salinity and chlorophyll a have more effect on the mortality of two-year-old oysters (respectively 5% and 4% of the variance)