The wide range of data from the French Ifremer/REMORA network, collected since 1993 allow a global characterization of mortalities of 1-year old and 2-years old cupped oysters Crassostrea gigas, that are cultivated in the main French oyster areas (Fleury et al, 2003), and thus a better understanding of the causes of these mortalities (Ifremer /MOREST project). The average annual mortalities on intertidal sites range from 10 to 20%. Most of the mortalities take place in Spring and Summer. 1-year oyster mortalities occur in Marennes and several sites in Brittany whilst 2-years oyster mortalities occur in the same sites and in baie des Veys (Normandy). In addition, 2-years oyster mortalities appear more variable according to different annual surveys. Internal (physiological) factors and external (environmental) factors have been considered successively. A Principal Component Analysis (PCA) on seasonal mortalities, growths and gonad maturations showed that, at the time scale of the REMORA monitoring (seasonal data), no strong correlation can be set up between mortalities and growth or maturation. 2-factors ANOVAs (Sites and Years) on the Spring+Summer mortalities differentiated the variance weights of these factors according to the year-class of animals : the site effect was slightly higher (51%) for the 1-year class, whereas the year effect was higher (74%) for the 2-years class. This was reinforced by the result of another PCA that included meteorological and environmental data such as rain, sun, air and water temperature and chlorophyll a, which displayed that the 1-year oyster mortalities were the most correlated (even if not significantly) with chlorophyll a and temperature (which are site dependant), whereas the 2-years oyster mortalities had a closer correlation to the rain (year dependent). In conclusion, this survey reveals more clearly the effect of external factors than internal ones on oysters mortalities. It also reveals that the causes of mortality may be somewhat different for 1-year oysters (preponderance of geographical factors) than 2-years oysters (meteorological factors). (Soletchnik et al, 2007). References : Fleury P.G., Le Ber E., Claude S., Cornette F., d'Amico F., Guilpain P., Palvadeau H., Robert S., Le Gall P., Ropert M., Simonne C. & Vercelli C., 2003. Comparison of Pacific oyster (Cr. gigas) rearing results (survival, growth, quality) in French farming areas : a 10-years monitoring (1993-2002) of the network Ifremer /REMORA. 95th Annual meeting of the National Shellfisheries Association, New-Orleans (U.S.A.), 13-17 Avril 2003. Soletchnick P., Ropert M., Mazurié J., Fleury P.G. & Le Coz F., 2007. Relationships between oyster mortality patterns and environmental data from monitoring networks along the coasts of France. Aquaculture 271,1-4: 384-400