he aim of our studies was to verify the hypothesis that heterotrophic nanoflagellates (HNF) prefer large and metabolically active bacterial cells, and avoid small and inactive bacteria. Determined graz-ing rates on bacteria differing in sizes and metabolic activity and observed changes in bacterioplankton structure in samples with and without bacterial grazers indicated that HNF prefer large (but not too large) and actively metabolizing bacterial cells. Bacterial size fraction between 0.4 and 0.8 µm represented the majority of all grazed bacteria, i.e. 61.6ą6.9 %. Grazing rates on live bacteria were 1.83 times greater than grazing rates on dead bacteria. This preferential protozoan feeding strongly affects composition and activity of bacterial communities in aquatic environments.