Bacterivory of pelagic rotifers and cladocerans in eutrophic Lake Loosdrecht (The Netherlands) was determined by microscopic observation of in situ tracer particle uptake. Ingestion rates of rotifer species using 0.51 μm microspheres or fluorescently labelled bacteria as tracers differed, with one exception. The ingestion rates depended on both the species and the tracer type. For cladocerans, fluorescently labelled bacteria seemed to underestimate grazing, presumably due to rapid digestion of tracer cells. Comparing results obtained with 0.51 μm microspheres, rotifers were much more important grazers on bacteria than cladocerans in the study period (April-September). Based on microspheres, the rotifer populations with the highest uptake of bacteria were Filinia longiseta (May-July) andAnuraeopsis fissa (June-September). According to the uptake of fluorescently labelled bacteria, Conochilus unicornis had the highest uptake in June and A.fissa in July.