The 1995–1998 studies on the unionid fauna of the Pilica catchment area (C Poland) included 103 localities in the river, tributaries, floodplain water bodies and the dam reservoir on the river. As a result of qualitative and quantitative sampling six unionid species (one represented by two forms) were recorded: Unio crassus Philipsson, U. tumidus Philipsson, U. pictorum (Linnaeus), Anodonta cygnea f. cygnea (Linnaeus), A. cygnea f. cellensis (Schröter), A. anatina (Linnaeus) and Pseudanodonta complanata (Rossmässler). Three of them: U. crassus, A. cygnea f. cygnea and P. complanata were found to be rare. Dominance structure and density of unionids are discussed. Spatial distribution and abundance are analysed on the background of such ecological factors as character of bottom sediments, bottom configuration, vegetation, distance from shore, seston content as well as physico-chemical, hydrobiological and bacteriological water pollution. Ecological preferences of each species are described. Selected populations are characterised with respect to their age structure. Species which were abundantly represented are characterised with respect to their metric characters and sexual dimorphism, as well as effect of zebra mussel on their growth and condition.