ABSTRACT. – 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 differences 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 (remaining organs) clearly exhibited different isotopic enrichment levels, with δ_Mu > δ_Gi > δ_Re regardless of C or N elements, culture sites and seasons. δ¹⁵N discrimination between organs was rather 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.