Natural abundance, 13C/12C ratios (δ13C) of Californian sea hares, Aplysia californica, (Gastropoda: Opisthobranchia) were measured for comparison with the animals preferred red algal diets, Plocamium cartilagineum and Laurenciapacifica. Isotopic compositions of animals collected from around Santa Catalina Island, did not reflect those of the favoured red algal diets, nor the majority of algal species found in the field. However, animals held in seawater tanks and fed diets with a constant carbon isotopic composition exhibited δ13C values similar to their food. Fast turnover tissue, such as the muscle tissue of actively growing juveniles or egg masses laid by adults, showed δ13C values within ±2‰ of the algal diet. Slow turnover tissue, such as the muscle tissue of large adults, reflected the diet over several months. Stable carbon isotope ratios thus proved a useful tool to clarify the extent to which specialized feeding in A. californica can occur upon algae which are only moderately to rarely common.