The genetic analysis of reproductive isolation between species of Drosophila has now reached the resolution necessary to start answering one of the fundamental questions of evolution: what is the genetic basis of species differences? A.H. Sturtevant, one of the founders of Drosophila genetics, was fascinated by this question and thought he had found a way to analyse it when he realized that 'Drosophila melanogaster' was actually two species: D. melanogaster and D. simulans. By passing genes between these two species he hoped to investigate their genetic differences directly. No doubt he was disappointed to find that the D. melanogaster/D. simulans hybridization resulted only in unisexual sterile hybrids, a disappointment appreciated all the more by modern evolutionary biologists. Seventy-five years after Sturtevant's description of D. melanogaster/D. simulans hybrid sterility, we have discovered a strain of D. simulans that produces fertile female hybrids in crosses with D. melanogaster. Our discovery promises to bring the enormous resolution of D. melanogaster genetics to the study of reproductive isolation and species differences.