Effects of the grazing activity of the gastropod Lymnaea elodes (arctica?) on animal community structure in the rocky littoral of an arctic lake were examined. In situ caging experiments on natural substrates with various snail densities resulted in shifts in faunal biomass abundance and distribution. Chironomids living in fixed tubes (Paratanytarsus) were greatly reduced at high snail loads. Free—living scrapers (Corynoneura, Crictopus cf. sylvestris, Zalutschia trigonocies, and others) fared only slightly better, while predators (Arctopelopia and Ablabesmyia) were relatively less sensitive to changes in snail density. Trichoptera larvae, Acari, and copepods were unaffected by the snails. Ostracoda and Cladocera biomass was significantly reduced by Lymnaea. All of the less sensitive groups were relatively more mobile. Reduced algal biomass and chlorophyll concentration were correlated with increased snail densities. Lymnaea production was limited by conspecific density and food.