The population of V. viviparus (L.) was studied at 16 sites situated along the mid and lower sections of the Narew River, with respect to the population density, sex ratio, size structure and fecundity. The density ranged from 50 to 600 indiv./m². The females comprised 53–90% of population, and represented mainly the largest size classes (III and IV), while most of the males were of size class II. The smallest snails (class I) were found only among the males (up to 50%). In all the sites gravid females predominated, being the most abundant in size class IV (80–100%). The average number of embryos per female was 18 (range 1–59) and increased with the shell height and body mass.