Intertidal grazing gastropods play an important role in structuring rocky shore communities. For many years the main food resource to these animals was considered to be epilithic biofilms. This study shows that macroalgae are a more important component of gastropod diets than was previously appreciated and that biofilms contribute surprisingly little to their trophic ecology. Stable carbon and nitrogen isotope analyses were used to examine grazer diets, comparing sources of assimilated carbon and nitrogen in gastropod tissues between moderately wave exposed and sheltered sites in northern and southern Britain. Carbon isotope (δ13C) values of biofilms were considerably lower than those of grazing gastropods at all of the sites examined. The δ13C values of grazer tissues were much more closely related to the carbon isotope values of macroalgae, demonstrating the importance of these foods to intertidal grazing gastropods. Nitrogen isotope values were consistent with this conclusion and reflected differences in the source pools of nitrogen between the two sampling regions. Gut contents analysis was used to directly quantify the diet of the intertidal limpet Patella vulgata in detail. Biofilms and corticated/leathery macroalgae contributed most substantially to patterns of ingested materials and limpet diet was strongly influenced by food availability. Animals from sheltered and northern sites consumed more macroalgae whilst animals from southern sites consumed more biofilm. Stable isotope data from mensurative experiments confirmed the important role of food availability in controlling limpet diets. Data from manipulative caging experiments provided further evidence for this effect and also showed that species identity and interspecific competition influence trophic ecology in intertidal gastropods. Patella depressa isotope values remained different from those of Patella vulgata irrespective of food availability and competition in experimental cages. Stable isotope ratios of Patella vulgata and Littorina littorea, distinct in single species cages, converged towards a common value when held together.