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 delta C-13 and delta N-15 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 delta C-13 values between sites partly due to higher investment in gametogenesis for the oysters in BDV than in RDB; these differences widened when lipid normalized delta C-13 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 delta(Mu) > delta(Gi) > delta(Re) regardless of C or N elements, culture sites and seasons. delta N-15 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 delta C-13 values in bivalves.