Genetic analysis of male sterility in Drosophila virilis x D. lummei hybrids was performed. It was shown that hybrid male sterility was caused mainly by incompatibility of sex chromosomes X and Y and homozygous chromosome 2 of D. virilis with other autosomes of D. lummei. Genes of chromosomes 3 and 5 enhanced sterility whereas genes of chromosome 4 suppressed its development. Factors controlling hybrid male sterility were demonstrated to have a recessive type of inheritance. At 17 degrees C, chromosome 6 of D. lummei was eliminated in F1 hybrids of the cross female D. virilis x male D. lummei. At 17 degrees C and 25 degrees C, sterility of the F1 hybrid males was 96% and 9%, respectively. Thus, two isolation mechanisms-elimination of chromosome 6 and male sterility-can simultaneously operate in F1. In addition to the region of the species-specific inversion In 1(a+b), factors controlling male sterility were mapped to the vermillion region of the X chromosome.