Changes in the survival rate in adults and embryos of the pond snail Physa acuta were studied after acute whole-body gamma irradiation. The LD50 value of the adult snails was about 40 kR. The LD50 values of the embryos irradiated 0 and 1 day after oviposition were about 0.9 and 2 kR, respectively. Histological changes in the ovotestis, the number of eggs laid, and their hatchability were examined in the irradiated adult snails. A fall and a subsequent recovery were observed for these characteristics after irradiation with 8 kR of gamma rays. The relative constitution of the germ-cell populations was greatly changed by the same dose of gamma rays. The proportion of immature germ cells was reduced, and the total number of germ cells also diminished 11 days after irradiation. After depletion, the ovotestis was first repopulated with gonia , and then with oocytes, spermatocytes, and spermatids.