Abstract The grazing activity of the Antarctic prosobranch gastropod Laevilacunaria antarctica on macroalgae was investigated in order to evaluate its position in the shallow water food web of Potter Cove, King George Island, Antarctica. From 12 abundant macroalgal species tested in feeding experiments, L. antarctica avoided only three species, the brown algae Ascoseira mirabilis, Phaeurus antarcticus, and Himantothallus grandifolius, significantly. The gastropod can be classified as a general feeder on macroalgae in the rocky intertidal and shallow waters of Potter Cove. Comparative grazing experiments showed that epiphytic diatoms contribute about 2/3 to the total diet of L. antarctica. Mean abundance and biomass of the snails in this area were 292.0±135.3 (S.D.) ind m −2 and 1.549±1.021 (S.D.) g AFDW m −2, respectively. The total consumption of macroalgae by the population of L. antarctica in Potter Cove is estimated to be 118 kJ m −2 year −1. This alone, however, is not sufficient to sustain the standing stock of gastropods in the investigation area. Grazing experiments showed that feeding on macroalgae which carry epiphytic diatoms is about three times higher than on macroalgae without epiphytes. It is therefore concluded that the population of L. antarctica in Potter Cove is maintained by consumption of both epiphytic diatoms and macroalgae. Feeding experiments showed that chemical compounds of macroalgae are not active in a chemical defense against grazing of L. antarctica. It is therefore discussed that structural characteristics of the algae can explain the low grazing rate of L. antarctica on some macroalgal species. Various models explaining macroalgal–herbivore relationships based on functional groups of algae are considered. The predictions on the feeding of L. antarctica derived from these functional form models in many cases correspond with the results derived from feeding experiments in this study. They are, however, inconsistent regarding grazing on or avoidance of some algal species and thereby obscure important information on special adaptations of organisms, e.g. L. antarctica, in macroalgal–herbivore interactions.