Lacustrine sediments of the lakes Wigry and Białe Wigierskie (Wigry National Park), up to ca. 2.5 m thick, are developed as lacustrine chalk intercalated with sand, mud and locally overlayed by peat. They contain rich mollusc assemblages comprising taxa typical of lakes and other permanent water bodies. Shallow water and flowing water species are accessory components of the fauna; land snails are represented by single specimens. Dreissena polymorpha (Pallas) and Potamopyrgus antipodarum (Gray), found in upper parts of the profiles, indicate a recent age of the sediments. Lacustrine chalk and calcareous gyttia are typical deposits of the Late Glacial and Early Holocene; in the Upper Holocene they developed only in very few lake basins, and the studied lakes are exceptional. The lithological and malacological analysis of their deposits makes it possible to reconstruct sedimentary processes controlling the accumulation of lake-filling calcareous deposits that are widespeared in N Poland.