AbstractMale sterility is one of three phenotypic effects of mutations at three loci in region 9F4 to 10C2 of the polytene X chromosome of Drosophila melanogaster, female sterility and reduced viability being other mutant effects. The male sterile (ms) alleles at these loci are allelic with lethal mutations. Male sterile mutations are induced at lower frequencies than lethal mutations at these loci by EMS. Because gene products of two of the loci are essential to larval development and the product of the other locus is essential to pupal development, the occurrence of ms mutations is not restricted to loci whose products function solely in developing adult tissue. Mutations at a locus in 10A2‐5 cause male sterility without adversely influencing viability or female fertility. We conclude that many loci whose products function in nonreproductive tissue can mutate to a form for which male sterility is a secondary effect. Another group of loci encode products that function exclusively in reproductive tissue.