The cytogenetical effects of long-term irradiation in embryo tissue of gastropod snail (Lymnaea stagnalis L.) as chromosome aberration rate as well as change of structure, level of differentiation, death and cytogenetical stability of hemolymph cell population has been studied. Our researches were carried out during 1998–2007 in water bodies within the Chernobyl Exclusion. The absorbed dose rate for snails from research water bodies within the exclusion zone was registered in the range from 1.8 mGy/year to 3.4 Gy/year. The highest rate was found in snails from lakes, the lowest – from the river ecosystems. Molluscs from the control lakes were characterized by absorbed dose rate about 0.3 mGy/year. The lowest rate of chromosome aberration was determined in molluscs form the control lakes – 1.1–2.0%. The highest rate was found in snails from lake ecosystems – 21–23%. In hemoliph of snails from contaminated lakes of the exclusion zone the quantity of death cells averages 40%, the part of phagocytic cells averages 42% as well as decrease of the young amoebocytes quantity to 10–20.1%. In the control water bodies the part of death cells averages from 2.2 to 5.3% and the quantity of phagocytic was at level 3.0–4.2%. The quantity of young amoebocytes have useful increased here to 79.7–89.6%.