The phytoplankton community of south Lake George, New York, has recently undergone a dramatic shift in composition; from a community dominated by Chrysophytes, Cryptomonads, and Chlorophyta (1975–1976) to one currently dominated by blue-green algae, i.e.Anacystis incerta and Aphanothece nidulans. No increases in nutrient concentrations or inputs have been documented before or during this period. This shift in dominance can be related to changes in higher trophic levels, i.e. grazers and planktivores. Standing crop and abundance of the small-bodied filter feeders, Bosmina longirostris, Daphnia galeata, D. dubia, Holopedium gibberum, Diaptomus minutus and D. sicilis are significantly greater in the south basin. Standing crop and abundance of the large-bodied Crustacea, Daphnia pulicaria, Epishura lacustris and Mysis relicta, are significantly greater in the north basin. The clutch sizes of all herbivorous species except D. minutus were significantly greater in the south basin populations. These differences are consistent with greater productivity and size slective planktivory in the south basin. Stomach analysis of the recently introduced rainbow smelt, Osmerus mordax indicates a marked selection for the large-bodied Crustacea. The establishment of large populations of rainbow smelt in the south basin of Lake George is responsible for significant basin differences in the abundance of large-bodied Crustacea and appears to have contributed to the changes in phytoplankton community composition. The shift to small-bodied Crustacea in the south basin has resulted in significantly lower grazing rates but generally higher P release rates in the south basin. These factors contribute to greater springtime phytoplankton production and silica depletion in the south basin. Coccoid blue-green algae are able to dominate waters with low phosphorus and silica concentrations, i.e. Lake George. Thus, the establishment of rainbow smelt in Lake George coincides with, and appears to be responsible for, changes in phytoplankton community composition.