The feeding behaviour of the zebra mussel (Dreissena polymorpha) was studied in the laboratory on different combinations of food, including a green alga Chlamydomonas reinhardtii) and toxic and non-toxic strains of the cyanobacterium Microcystis aeruginosa. 2. The highest clearance rate of phytoplankton by zebra mussels was found when the mussels were feeding on a mixture of Chlamydomonas and non-toxic Microcystis, the lowest on a mixture of Chlamydomonas and toxic Microcystis. 3. The differences found in the clearance rates between food combinations can be partly explained by the production of pseudofaeces containing live phytoplankton cells. Zebra mussels expelled significantly more live phytoplankton cells in the presence of toxic Microcystis than in the presence of non-toxic Microcystis. The pseudofaeces contained predominantly live Chlamydomonas cells. Proportionately much less live Microcystis cells were encountered in the pseudofaeces. 4. Consequently, grazing of zebra mussels on a combination of Chlamydomonas and Microcystis may finally result in a dominance of Chlamydomonas over Microcystis. The presence of toxic Microcystis may even strengthen this shift.