We studied the influence of food availability on the growth (whole body and organs) of the oyster Crassostrea gigas and on the dynamics of their δ13C and δ15N respective values. Juvenile oysters originating from Arcachon Bay were transplanted to two contrasting ecosystems, Baie des Veys (BDV) and Rade de Brest (RDB), for a 1-yr growth survey. In BDV, chlorophyll-a concentrations ([Chl-a]) was 3 times higher than in RDB on average, which accounts for the differences in oyster growth between BDV and RDB. Differences in trophic conditions could also explain the differences in C/N ratios and δ13C values between sites partly due to higher investment in gametogenesis for the oysters in BDV than in RDB; these differenc- es widened when lipid normalized δ13C values were used. Oysters clearly selected microalgae as the main food source, and especially phytoplankton. Gi (Gills), Mu (Muscle) and Re (remain- ing organs) clearly exhibited different isotopic enrichment levels, with δMu > δGi > δRe regard- less of C or N elements, culture sites and seasons. δ15N discrimination between organs was rath- er equivalent between sites. These results would benefit from an experiment under controlled conditions along a calibration of the lipid normalization to correct δ13C values in bivalves.