The species structure of epizoic communities of Rotifera was analyzed on three species of freshwater bivalves: Anodonta anatina (Linnaeus), A. cygnea (Linnaeus) and Unio tumidus (Philipsson) from lakes and rivers of north-eastern Poland. Epizoic rotifer communities were abundant and relatively rich in species. The communities were mostly built of common littoral species, like Colurella uncinata, Lecane flexilis, Lepadella quadricarinata, Ptygura melicerta. They were accompanied by species typical for pelagic waters (e.g. Trichocerca pusilla, T. similis). Bivalves inhabiting outflows of rivers from Knyszyn Forest had the most diversified and abundant epizoic communities. Large differences were also observed as regards particular species occurrence in different habitats. Maximum numbers of Ptygura melicerta living on the surface of bivalves from rivers accounted for 150 individuals per bivalve specimen, whereas those living on animals from lakes - up to 10 individuals. All studied bivalves were colonized by very large numbers of bdelloids.
