Ciliate picoplanktivory was studied in two different freshwater systems with abundant autotrophic picoplankton (APP): the eutrophic Rimov Reservoir (South Bohemia) during the late summer of 1993 and the oligo- to mesotrophic lake Piburger See (Tyrol) during May 1994. Picoplankton were sized by an image-analysis system and species-specific grazing rates of ciliates on bacteria and APP were determined using fluorescently labelled prey. According to the grazing data (as the mean uptake rate of picoplankton ind.(-1) h(-1)), ciliates were split into three ecological groups in order of their decreasing picoplanktivory and increasing significance of nanophytoplankton in their diet. (i) Highly efficient fine suspension feeders consisting of vorticellids (4200 bacteria, 560 APP), four oligotrichs and Cyclidium sp. (380-1580 bacteria, 57-210 APP), with 2 mu m. (ii) Less efficient fine suspension feeders, coarse filter feeders and detritophages consisting of Cinetochilum margaritaceum, Pelagostrombidium fallax, Cyrtolophosis mucicola and Coleps spp. (60-173 bacteria, 2-27 APP), with 7-32% of individuals ingesting algae >2 mu m. (iii) Raptorial feeders consisting of four prostomatids belonging to the genera Urotricha and Balanion, with a negligible importance of picoplanktivory (8-61 bacteria, 0.2-14 APP). With the exception of one prostomatid, >58% of individuals ingested algae >2 mu m. Grazing data for the six species from the first group were converted into organic carbon. Assuming a 35% gross growth efficiency, the calculated potential doubling times were between 29 (Halteria grandinella) and 43 h (an unidentified oligotrich), except for the mixotrophic Pelagohalteria viridis (118 h). The doubling times estimated from changes in ciliate abundance for four out of the six species were very close to the calculated values, indicating that this group of heterotrophic ciliates can meet all of its carbon requirements by feeding exclusively on picoplankton.