Density, sex ratio, size structure and reproduction were analysed in populations of Viviparus contectus (Millet) from oxbows differently connected with the Bug River. The mean density was 20 ind./m2, the sex ratio was 1:1. Snails of size classes II (8.1-12.0 mm shell width and height), III (12.1–25.0 mm width and 12.1–20.0 mm height) and IV (25.1–35.0 mm and 20.1–35.0 mm) were the most numerous. The youngest snails (shell heightand width < 8 mm) were the least numerous. Smaller snails (class II) dominated in the spring. The proportion of classes III and IV increased in the summer and autumn. The number of embryos per female was the highest during the spring and summer - the period of intense reproduction. The number of embryos per female was the smallest (mean 6.8) in the oxbow isolated from the river; it was higher in the flow oxbows (13.6 and 14.2). In the isolated lake that number increased with the female's shell size. In the flow lakes the number of embryos washigh irrespective of the female's size; females from these lakes showed the highest fecundity in all size classes. Int he spring the mean number of embryos per female in classes II (11.5), III (12.4) and IV (14.6) was higher than in the largest females from the isolated lake. Similarly, in the summer the mean fecundity of females in class II(15.4 embryos/female) was higher than in the largest females in the lake isolated from the river (6.2).