Ingestion of fluorescent particles by natural protozoan assemblage was studied in the Rimov Reservoir (Southern Bohemia) from April to October, 1987. Attached and free-living bacterial abundance, proportion of active bacteria, density of suspended particles and biomass of cladocerans were also monitored. Heterotrophic nanoflagellates (HNF; 5-12.8 10(2) ml-1) were the dominant bacterial micrograzers during the spring period and consumed 3 to 9 % of the total bacteria per day. After the spring phytoplankton bloom maximum densities of suspended particles and attached bacteria (up to 28 % of the total counts) were found. Development of cladocerans in May sharply decreased the proportion of attached bacteria and kept them below 5 % of the total counts. All the studied components of plankton except Cladocera decreased during the clearwater phase. The most significant drop was observed in the numbers of protozoans, and they were negligible for bacterial elimination. Bacterial losses during that time apparently were due to cladoceran grazing. During the summer period, ciliates (15-142 ml-1) were mostly dominant micrograzers, and protozoan community grazing increased up to 21 % of bacterial standing stock per day. The proportion of active bacteria was strongly correlated with protozoan grazing (r = 0.83).