Drosophila willistoni and its subspecies quechua are morphologically indistinguishable, but differ in relative frequencies of certain allozyme genes. The cross quechua female X willistoni male produces nearly or completely sterile males, while the reciprocal cross gives fertile males. Hybrid females are fertile. Analysis of the sterility with the aid of chromosomes marked with mutant genes shows that the second and X-chromosomes have major and third chromosomes minor effects. Backcross males of the same chromosomal constitution may be either fertile or sterile; the threshold effect may be due to environmental variations or to gene variants present in the strains crossed. Only a trace of ethological isolation between the subspecies is present. In contrast to a narrowly localized and geographically isolated subspecies bogotana of D. pseudoobscura, the subspecies quechua of D. willistoni is fairly widespread, although its geographic area is not accurately known. Neither with bogotana nor with quechua are there valid reasons to suppose that these subspecies are of very recent origin.