The present work aims at establishing the toxic effects on spermatogenesis of a single injection of the organophosphorous agropesticide Parathion, administered to immature male mice. Seven days old animals were injected intraperitoneally with 1/3 the LD50. Body and testicular weights were measured. Quantitative as well as qualitative analysis of seminiferous tubules and germ cell populations were done for control and injected mice 8, 16, 28 and 50 days after injection. The cytotoxic effect of the pesticide is substantiated by a decrease of body and testis weight and an early damage of germ cells in treated mice. The effect is reversible and recovers at longer intervals