The genetic elements that control sexual isolation and subsequent speciation are not well characterised. In the June 5 Proceedings of the National Academy of Sciences, Doi et al. describe the identification of a Drosophila locus for female mate discrimination that causes reproductive isolation (Proc Natl Acad Sci USA 2001, 98:6714-6719). They studied Drosophilaananassae and Drosophila pallidosa, which are sexually isolated species. Courtship song generated by male wings influenced female choices of mating partners. Genetic analysis of F1 hybrid females allowed Doi et al. to map a female discrimination locus on chromosome 2, and chromosomal introgression analysis using extensive backcrosses refined the mapping to near the Delta gene locus.