We investigated ontogenetic and seasonal changes in the feeding ecology of 2 small pelagic fish species, the European anchovy Engraulis encrasicolus and the European pilchard Sardina pilchardus in the Gulf of Lions (NW Mediterranean). By analysing the stable isotopes delta C-13 and delta N-15, we determined the seasonal variation in the food sources and in the trophic level of these species, and we examined dietary shifts during development. The results of these investigations provided estimates of the diets of both species. We compared the values observed during different seasons (summer, autumn and winter) and at different developmental stages (late larvae, juveniles and adults) for both species, together with the values of potential groups of prey (microplankton, cladocerans, copepods and appendicularians). Late-larvae preferred to feed on microplankton, although differences in the diet appeared after metamorphosis. Cladocerans were usually the preferred prey when available (summer), and appendicularians were the preferred prey in autumn. During the winter, the diets seemed to be more heterogeneous. Different feeding behaviours between the late-larvae of the 2 species were the most likely reason for the slightly different trophic levels found in the present study. This research demonstrates that studies with stable isotopes can furnish an alternative and/or complementary method for determining the diet of small pelagic fishes over extended periods and provides comprehensive knowledge of the functioning of the pelagic ecosystem.