To establish the effects of food quality on the carbon and nitrogen budgets, experiments with the mussel Mytilus edulis and the cockle Cerastoderma edule have been performed. Clearance rates, absorption efficiencies, oxygen consumption and ammonia excretion have been measured as a function of diets composed of Phaeodactylum tricornutum and suspended sediment. Net carbon loss is observed at high particle concentrations, probably due to mucus production. Nitrogen budgets are less influenced by high seston concentrations. Animals are able to maintain a constant N-absorption by means of selective ingestion.