Grazing rates by a zooplankton community were measured in situ by a radiotracer cellmethod at depths of 1 m and 4 m at the upper tidal freshwater portion of a regulated river over a year.The objectives were to evaluate the likely grazing impact on the river phytoplankton community and toproduce predictive models by regressing the measured grazing rates against zooplankton biomass,temperature and food concentrations (represented by chlorophyll a). Grazing attained rates (overallaverage 0.2 day-', range 0.01-0.59 day-', expressed as instantaneous mortality rates of algal cells)comparable to those reported for lentic zooplankton communities. The measured community grazingrates were predictable largely as a function of total biomass or rotifer biomass and surface temperaturefor 1 m depth, and as a function of total biomass or juvenile copepod biomass and surface temperaturefor 4 m depth, with all-positive regression coefficients in the models. Owing to the predominance ofmicrozooplankton in the river, the impact of zooplankton community grazing appears likely to be linkedto a small-size fraction of the phytoplankton community all year. Management strategies for river waterquality may need to take account of possible functional demarcation of grazing by river zooplankton.