Spatial distribution and seasonal dynamics of molluscs in the Liwiec River (South Podlasie Lowland) were studied: nineteen gastropod species were found, with members of Planorbidae (7 species) and Lymnaeidae (5 species) dominating, and other families being represented by single species. Of bivalves three unionid species, and 2 genera of Spheriidae were represented. The species diversity varied along the river: the malacocenosis of its upper section was the most diverse (14 species), while fewer species (10) were found in the river mouth region. The mid section was the least diverse (5 species). The mollusc densities varied from 1 to 37 indiv./m². They were the highest in the upper section (11–33 indiv./m²), and at one site in the lower section (37 indiv./m²); the lowest densities were observed in the mid section (1–7 indiv./m²); in the river mouth region they ranged from 4 to 16 indiv./m². The mollusc abundance was higher in June and September than in April.