The results from two experiments on induced crossing-over in the germ cells of adultDrosophila melanogaster males, heterozygous for 12–13 third-chromosome mutant loci, support the premise that the F1 offspring recovered in the 6–8 day broods are derived mainly from cells that had been primary spermatocytes at the time of irradiation. This conclusion is also rather well supported by the results from similar experiments reported in the literature. The frequency of exchange in the third-chromosome following irradiation of the adult male is in the order of 0.01% for the 1–3 and 4–5 day broods, 0.15% for the 6–7 day broods and 0.31% for the 8th day brood in the pooled data from 18 experiments. The spontaneous rate for the third-chromosome is in the order of 0.001%.