EN
The aggregations of V. viviparus were observed in the littoral of Zegrzyński Reservoir (Central Poland), and in the outfalls of its tributaries. They were present over five years of studies (1990-1994), at the same sites. The densities of snails in the aggregations differed among the sites (from over 800 to 500 ind. m⁻² in the reservoir itself), and showed seasonal changes with maximum in summer, and minimum in spring and fall. Swarming of the snails was related to the environmental changes and biology of this species. Tendency to form bigger snail conglomerations in the mouth sections of the rivers results from continuous inflow of organic matter being food supply for these organisms.