The Liwiec River, the biggest left-bank tributary of the Bug River, drains South Podlasie and Middle Mazovian Lowlands. This study aimed at tracing longitudinal changes in the composition, diversity, dominance pattern and abundance of its mollusc assemblages. Special attention was paid to the role of spatial disturbances in the river continuum, of both natural (tributaries) and anthropogenic origin (sewage treatment plants, small dam). Forty two mollusc species were recorded, with the respective values within the upper (UR), middle (MR) and lower (LR) river sections of 27, 19 and 34. The composition of mollusc assemblages showed some longitudinal changes, but without any clear progression along the river course. However, some differences among the malacofaunas of UR, MR and LR were observed, including large variation in species richness along the river. A relatively small species richness (less than 5 species) was found at 26% of the sites. The rather high total species diversity (γ) resulted mostly from considerable differences in the species composition among the sites. The effect of spatial discontinuities in the river course on the mollusc assemblages was rather weak, but they were more visible than the differences between consecutive sites within free-flowing river sections.