A dialysis bags experiment, involving the use of natural water from eutrophic Lake Miko8ajskie (Mazurian Lake District, Poland), was performed to assess the impact of grazing pressure by heterotrophic nanoflagellates (HNF; <10 μm) on bacterial abundance, their size distribution, tax- onomic composition and activity measured by bacterial secondary production (BP) and the contribu- tion of active bacteria with an intact membrane (MEM+). Three different techniques — size-fraction- ation (<1.0 μm pore size), dilution (10×) and selective eukaryotic inhibition (cycloheximide and colchicine) — were applied to control HNF grazing. There were significant differences in the effec- tiveness of grazing pressure limitation between the different techniques employed. The most distinct changes in the bacterial community were found in the control and the 10× diluted treatments; in both, a considerable increase in HNF abundance was noted. In these 2 treatments, a significant decrease in both bacterial numbers and contribution of MEM+ cells, as well as an increase in the proportion of bacterial filaments (up to 50 μm long), was observed. In contrast, the use of cycloheximide and colchicine in combination resulted in the lowest and an almost constant HNF abundance. In addition, high numbers of bacteria with a large proportion of MEM+ cells (30 to 40%) and exponential increase of bacterial secondary production were found. These results suggest that eukaryotic inhibition is the most useful method of limiting HNF grazing pressure and involves minimal manipulation of the water sample. Close relationships between bacteria and HNF confirm the crucial role of nanoflagellates in shaping bacterial abundance, morphology and activity.