A DNA fragment that includes the wild-type rosy (ry+) gene of Drosophila melanogaster has been introduced by microinjection into the germ line of the reproductively isolated species Drosophila simulans and incorporated into the D. simulans genome. Transformation was mediated by the transposable element P, which occurs in the genome of most natural populations of D. melanogaster but not in D. simulans. Rubin and Spradling [Rubin, G.M. & Spradling, A.C. (1982) Science 218, 348-353] have previously shown that the ry+ DNA fragment, which is flanked by recognition sequences of P element, can transform the germ line of D. melanogaster. Successful transformation in D. simulans indicates that the P element continues to function as a transposable element in the D. simulans genome. Moreover, the ry+ gene of D. melanogaster functions in the genome of D. simulans to produce normal eye color, despite the estimated 1 to 5 million yr of reproductive isolation since the evolutionary divergence of these species. Interspecific DNA transformation provides a useful method for the study of genetic differences affecting gene expression among related but reproductively isolated species.