Colonies of A. flos-aquae dominated the phytoplankton of a polymictic lake during a clear-water, potentially productive phase, coincident with and following a dense population of D. pulex. Intermittent breakdown of thermal stratification was correlated with initiation of A. flos-aquae populations. A. elenkinii occurred as single filaments and was restricted to summer periods of high turbidity and the absence or low concentrations of D. pulex. Enclosure experiments showed that D. pulex was largely responsible for the decline of the spring phytoplankton and the resulting clear-water phase. In enclosures subject to grazing, A. flos-aquae formed a substantial proportion of the phytoplankton biomass, whereas in the absence of D. pulex, A. elenkinii and other small phytoplankton dominated. The final biomass was a function of both the number of D. pulex and time. The critical density of D. pulex (without any correction for net efficiency) was 12-15 1-1, above which the rate of loss from the phytoplankton community exceeded the apparent growth. Above this critical density, A. flos-aquae formed most of the phytoplankton biomass and was apparently immune to grazing because of its large size. There was strong circumstantial evidence to suggest that A. flos-aquae was recruited from the sediments, but attempts to demonstrate this failed. Instead, Anabaena spiroides was shown to be recruited.