Studies on spatial diversity in species composition, dominance patterns and abundance of malacocenoses in the Zegrzyński Reservoir were carried out in the late 1990s, and changes since the 80s were assessed. The malacocenoses of the reservoir in the 90s were characterized by a considerable structural diversity and varied abundance. The species diversity was higher in the littoral while the abundance was generally higher in habitats located farther from the shore. After a period of rapid development of the 80s, in the 90s the malacofauna stabilised at a lower level of abundance and species richness, while the dominance patterns became more pronounced. The phenomenon could be related to progressive eutrophication as well as ageing of the reservoir. Compared to other dam reservoirs the mollusc fauna of the Zegrzyński Reservoir is still quite rich in terms of both abundance and number of species.