The malacofauna of the palaeolake deposits at Szymanowo (eastern Poland) was investigated. It represents the younger part of the climatic optimum of the Mazovian (Holsteinian) Interglacial (~MIS 11) and possibly the postoptimal period. The mollusc assemblage is composed of both standing and running water species, mostly connected with temperate climate. The presence of biostratigraphical indicators of the Mazovian, Viviparus diluvianus (Kunth, 1865), Lithoglyphus jahni Urbański, 1975 and Pisidium clessini Neumayr, 1875, is noteworthy. Variability in the structure and composition of the assemblage enables palaeoecological reconstruction. Changes in the water-level, vegetation and energy conditions are inferred from the malacological succession. Three stages of the lake development were distinguished. The first one is connected with deeper conditions and predominance of V. diluvianus and L. jahni. The second one, dominated by Bithynia tentaculata (Linnaeus, 1758), records a fall of the water-level and the growth of aquatic plants, evidenced by high frequencies of Valvata cristata Müller, 1774 and Acroloxus lacustris (Linnaeus, 1758). The third stage corresponds to another rise of the water-level and an increase in V. diluvianus, L. jahni, Valvata piscinalis Müller, 1774 and Pisidium henslowanum (Sheppard, 1823), which evidence some higher energy conditions.