We studied the functional response of the freshwater unionid bivalve Anodonta anatina, feeding on five phytoplankton strains differing in food quality: the small green alga Scenedesmus obliquus, a toxic and a non-toxic strain of the filamentous cyanobacterium Planktothrix agardhii and a toxic and a non-toxic strain of the coccoid cyanobacterium Microcystis aeruginosa. On S. obliquus, A. anatina had a type II functional response with a maximum mass-specific ingestion rate (IRmax) of 5.24 mg C g DW-1 h(-1) and a maximum mass-specific clearance rate (CRmax) of 492 (+/- 38) ml g DW-1 h(-1), the highest values for all the phytoplankton strains that were investigated. On toxic and non-toxic P. agardhii filaments, A. anatina also had a type II functional response, but IRmax and CRmax were considerably lower (IRmax 1.90 and 1.56 mg C g DW-1 h(-1); CRmax 387 (+/- 97) and 429 (+/- 71) ml g DW-1 h(-1), respectively) than on S. obliquus. Toxicity of P. agardhii had no effect on the filtration rate of the mussels. On the non-toxic M. aeruginosa (small coccoid cells), we also observed a type II functional response, although a type I functional response fitted almost as good to these data. For the colonial and toxic M. aeruginosa, a type I functional response fitted best to the data: IR increased linearly with food concentration and CR remained constant. CRmax and IRmax values for the (colonial) toxic M. aeruginosa (383 (+/- 40) ml g DW-1 h(-1); 3.7 mg C g DW-1 h(-1)) demonstrated that A. anatina filtered and ingested this cyanobacterium as good as the other cyanobacterial strains. However, on the non-toxic M. aeruginosa we observed the lowest CRmax of all phytoplankters (246 (+/- 23) ml g DW-1 h(-1), whereas IRmax was similar to that on toxic M. aeruginosa. The high maximum ingestion rates on S. obliquus and M. aeruginosa indicate a short handling time of these phytoplankton species. The high clearance rates on S. obliquus, toxic M. aeruginosa and P. agardhii reflect a high effort of the mussels to filter these particles out of the water column at low concentrations. The low clearance rates on non-toxic M. aeruginosa may be explained by the small size and coccoid form of this cyanobacterium, which may have impaired A. anatina to efficiently capture the cells. Although A. anatina had relatively high maximum clearance rates on non-toxic and toxic P. agardhii, this cyanobacterium does not seem to be a good food source, because of the observed high rates of pseudofaeces production and hence low ingestion rates.