Annually, during the months January to June, a heavy bloom of the dinoflagellate Peridinium cinctum fa westii occurs in warm (13–30° C), monomictic Lake Kinneret, Israel. The rates of carbon assimilation, determined with 14C, were not exceptionally high, but algal biomass and chlorophyll concentrations measured during the bloom season were among the highest recorded for the trophogenic zone of any natural water body. Effective utilization of environmental conditions by the motile Peridinium, which dominates the phytoplankton assemblage and is only slightly grazed, may partly explain the large standing crops. During the bloom, physiological activities are minimal (low assimilation numbers, activity coefficients, and specific growth rates); conversely, high values for these parameters occur in late summer and fall (when Chlorophyta predominate) and at the start of the bloom. The patterns of algal behavior in Lake Kinneret may have general implications for understanding the role of phytoplankton in warm eutrophic ecosystems.