A comparative assessment of physiological and cygotenetical parameters in Lymnaea stagnalis populations from Perstok lake and the inlet of Pripyat river with the highest in Belarus and moderate levels of radioactive contamination have been carried out. In 2004 f γ-activity of snails from river and lake amounted to 39.2 and 567.8 Bq kg-1, respectively, and β-activity - to 123.4 and 7055 Bq kg-1 respectively. Snails from the lake were characterized by the increase of the cells with cytogenetic damages (micronuclei) and by the decrease of cells with interphase death and proportion of young cells in hemolymph. It indicates the unfavourable environmental conditions in the lake due to increased level of radioactive contamination. Nevertheless, the embryonic mortality in the progeny of the snails from lake was lower and the average population radioresistance of snails much higher than that in the river indicating intensification of the radioadaptation processes in populations from the lake.