The fluorescent antibody (FA) technique was employed to estimate the filtration rates of Daphnia cucullata on the Alcaligenes eutrophus JMP134 strain introduced into lake water. We investigated the effect of animal density and food concentration on the filtration rates in 0.2-μm filtered and autoclaved lake water. The individual filtration rates were 0.08-1.12 ml animal^-1 h^-1. Filtration rates decreased with increasing grazer density and with increasing food concentrations. The grazing impact on indigenous and introduced bacteria was compared in experiments with 2-μm filtered lake water. Filtration rates of D. cucullata on A. eutrophus were greater than on indigenous bacteria. These differences were explained by size-selective grazing, implying that this mechanism is important in the elimination of introduced bacteria from lake water when the introduced bacteria are in the larger size-range of the indigeneuos bacterial flora.