Investigations of aquatic mollusc assemblages were conducted within a semi-natural catchment of a medium-sized lowland river Liwiec in five habitat types: the main Liwiec River channel, its secondary channels and six tributaries, as well as in natural ponds and man-made ditches within the river floodplains. To assess the contribution of each habitat type to the diversity, I used the species richness, diversity, rarity (species rarity index, SRI) and abundance, as well as the composition and structure of mollusc assemblages and compared them among the habitat types. The spatial pattern of mollusc diversity was analysed based on hierarchical partitioning. Over five years, 54 mollusc species were found, including three listed on the IUCN Red List or in Annexes II and IV of the EU Habitats Directive (Unio crassus Philipsson, Sphaerium rivicola (Lamarck) and Anisus vorticulus (Troschel)). The mollusc metrics in most cases did not differ significantly among the habitat types. All the habitats contributed relatively evenly to the mollusc diversity. The diversity at the site level was generated mainly by alpha component, whereas at the landscape scale habitat heterogeneity (beta component, i.e. β2) was very important. In order to maintain mollusc diversity, conservation efforts ought to focus specifically on the most heterogeneous fragments of the Liwiec River catchment.