Deposit and suspension feeders can switch feeding behavior and show variations in feeding mode as individuals pass through life-cycle stages. Stable carbon and nitrogen isotopes were used to trace changes in diet of the tellinid bivalves M. balthica facultative deposit feeder) and C. edule (obligatory suspension feeder), according to their size class. Analyses of variance showed differences in the δ 13 C between the species. C. edule showed a diet composed of microphytoplankton, whereas M. balthica could feed on a mixed diet of microalgae from benthos and plankton. Values of δ 13 C depended significantly on body size in M. balthica , providing evidence of ontogenetic variation in diet with small juveniles feeding entirely on microphytobenthos, while there was a gradual tendency for larger sizes to feed more on microphytoplankton. Therefore, although these species rely on different sources of food, large animals of M. balthica can overlap the trophic niche of C. edule . Population dynamics of the animals should be considered in food-web studies.