The frequencies of newly induced male-sterilizing lesions on both the X and Y chromosomes of Drosophila melanogaster were determined after either 4000 r of gamma-irradiation or adult feeding of ethyl methanesulfonate. The Y chromosome is approximately twice as sensitive as the X chromosome to newly induced male-sterilizing lesions after gamma-irradiation, but slightly less sensitive after ethyl methanesulfonate treatment. A large proportion of the radiation-induced lesions are associated with Y-autosome or X-autosome translocations, with the Y chromosome recovered in translocations far in excess of the frequency expected from metaphase lengths. Although translocations between the X and Y chromosomes or between autosomes do not appear to sterilize heterozygous males, interchanges between sex chromosomes and autosomes often sterilize males carrying them in a dominant manner, suggesting that the organization of the genome is critical for normal spermatogenesis. Complementation tests between recessive Y-linked male-sterilizing mutants do not reveal the existence of any additional fertility loci beyond the six previously defined.