The suspension feeding of Bithynia tentaculata was tested in laboratory experiments. The animals were fed in 1-1 aerated glass beakers, and filtration rates were calculated from changes in cell concentrations during the 6-h experiment. Temperature influenced the filtering rate, with minimum values of 5ml · ind-1 · h-1 at 5° C and maxima of 17.2 ml · ind-1 · h-1 at 18° C. Three food species of different size, motility and cell surface characteristics (Chlamydomonas reinhardii, Chlorella vulgaris and Chlorogonium elongatum) did not affect filtration rates. Suspension feeding increased with increasing food concentrations up to 12 nl · ml-1, above which feeding rate was kept constant by lowering the filtering rates. Even the smallest animals tested (<4 mm body length) were found to be feeding on suspended food at a rate of 2.7 ml · ind-1 · h-1, and increasing rates up to 8.4 ml were found in the 6-7 mm size class. All size classes of Bithynia showed a circannual fluctuation of their filtration rates. The ecological consequences of Bithynia's ability to switch between two feeding modes, grazing and suspension feeding, are discussed.