Young immature guppies and adult males are treated in dilute parathion, at doses of 0.01-0.1-1 and 10 p.p.m. At the 10 p.p.m. dose, fish die within a few hours, other concentrations do not cause a high mortality rate (fig. I) or alter spermatogenesis, if the exposure time is short (six days). On the contrary, if the fish remains in parathion over a prolonged period (40 days), there is a marked inhibition of spermatogenesis, expressed by a decrease in the number of germinal cells which is the greater, the higher is the dose of parathion. Results are similar to those obtained after hypophysectomy, which enables to suppose that parathion may play a role in the gonadotropic factors. These results confirm the necessity to define pesticide doses supportable in water, not using TLM but various criteria including gametogenesis.