Discrimination of inert and nutritive particles was investigated in five crustacean zooplankton species. The two particle types were represented by polystyrene beads and cultured algae respectively. The animals were offered combinations of beads and a 14C-labelled alga of equal size. Four combinations of different sizes were used (2,6, 11 and 19 μm). Bosmina coregoni strongly discriminated against the 11 and 19 μm beads, while particles of the two smallest sizes were ingested non-selectively within combinations. A selection against beads of all sizes was observed in Eudiaptomus gracilis although less intensively in the case of 2 and 6 μm particles. Daphnia cucullata, Chydorus sphæricus and Diaphanosoma brachyurum exhibited, on average, similar filtering rates for corresponding particles in the three smallest size classes but preferred the alga over the 19 μm bead. The two or three smaller sizes of beads may be used to assess filtering rates for similarly sized real food items. However, an uncritical use of this technique may be hazardous.