The plankton of nine Ontario lakes spanning several physiographic regions was sampled every two weeks during the ice-free period of 1981, and one lake was studied in the three previous years. Phytoplankton, zooplankton, and ciliated protozoa were sampled, counted and sized. The size data were converted to biomass estimates to yield quantitative comparisons of the relative allocation of biomass among different functional compartments. This is the first study to look simultaneously and quantitatively at the total plankton system of lakes (including ciliates, pbytoplankton and net zooplankton) over a broad physiographic region. Ciliates constitute −10% of the non-algal biomass and 5% of the total planktonic biomass of these lakes. Ciliate standing crops among lakes are significantly corrrelated with total organic and total inorganic carbon concentrations in the water column, while the dynamics of ciliate biomass fluctuations are significantly correlated with variations in total phosphorus concentration, in conductivity, in Kjeldahl nitrogen concentration, and in inorganic carbon content. There appears to be a significant dynamical relationship between ciliates as a proportion of the total planktonic biomass, exclusive of filamentous and large (>30 μm) spherical algae, and the relative biomass of small algae (2–5 μm) as a fraction of total algal biomass, again exclusive of filaments and large (>30 μm) algae. The hypothesis is advanced that ciliates primarily function as bacterial grazers in planktonic ecosystems and that their primary competitors in this role are rotifers.