Studies on the occurrence of molluscs in Lake Mikołajskie (Masurian Lakeland, NE. Poland) were carried out in 2011–2015. Samples were taken yearly from 5–10 sites in the littoral and sublittoral. Twenty four mollusc species were recorded, including 12 snails and 12 bivalves. Moreover, empty shells of another eight species were found. The most common species in most sites were snails Bithynia tentaculata (Linnaeus), Valvata piscinalis (O. F. Müller) and Potamopyrgus antipodarum (Gray), and bivalves Unio tumidus Philipsson and Dreissena polymorpha (Pallas). Live molluscs were recorded to the depth of 5 m and their mean density in particular sites ranged from 100 to 300 individuals per m². Compared to earlier studies, the number of species was markedly smaller which may be partly associated with the increasing trophic status of the lake. On the other hand, alien species (P. antipodarum, Lithoglyphus naticoides (C. Pfeiffer)) appeared in the lake in various periods of time and some species (e.g. Anodonta cygnea (Linnaeus)), not recorded for many years, returned to their sites. D. polymorpha had been studied permanently (with short breaks) for 50 years in the lake. Its abundance at the beginning of the 21st century was markedly higher than in the last two decades of the 20th century but far smaller than the enormous densities noted before. The live malacofauna differed greatly from the collection of empty shells in its species composition, dominance structure as well as in the age and size structure of selected species.