Mollusc communities of the Ohře (Eger) River, which drains a large part of northwestern Bohemia, were studied. In total, 43 freshwater mollusc species (22 gastropods, 21 bivalves) were found at 56 sites. The richest malacofauna (31 species) was recorded in the lower river section. One empty shell of Margaritifera margaritifera, a species regarded as critically endangered in the Czech Republic, was found. There are no previous records of this bivalve in the Ohře River and its tributaries, and its recent occurrence there is unlikely. Planorbis carinatus, Unio crassus, Pseudanodonta complanata, Pisidium amnicum and P. moitessierianum, endangered in the Czech Republic, were recorded. The population of U. crassus is the largest in northwestern Bohemia and one of the largest in the Czech Republic. It is divided in two parts by the Nechranice dam lake. Likewise, populations of other species are negatively affected by the dam, as well as by the pollution and watercourse alteration. In some sites the muskrat (Ondatra zibethicus) was observed to predate on unionids and especially on U. crassus. The occurrence of five alien species (Potamopyrgus antipodarum, Physella acuta, Gyraulus parvus, Corbicula fluminea, Dreissena polymorpha) was confirmed. A gradual invasion of C. fluminea was observed; the species is predated by an unidentified animal, probably also muskrat.