The occurrence of the snail Potamopyrgus antipodarum (Gray) in the waters of the Wigry National Park was studied, and the factors, affecting its distribution and dispersal, were analysed in 1997–1998. P. antipodarum was recorded from 7 out of the 24 examined water bodies of the Park and from the river Kamionka. The snail occurred in bottom deposits, on submerged and emerged macrophytes and in mats of filamentous algae in the lake Wigry, and clearly dominated in its malacofauna. In the remaining lakes, where it was present, it was not abundant. Active dispersal is not sufficient to explain the distribution of P. antipodarum in the lake Wigry. Its dispersal is probably a result of passive introduction by animals and humans, and to a lesser extent by water currents. An increased chlorophyll concentration in the pelagial has a positive effect on the occurrence of P. antipodarum. In much polluted habitats the vertical distribution and density of the species are limited. Significance of the trophy of water for the occurrence of P. antipodarum in the area is still unclear.