In flies, as in many other animals, mating does not guarantee successful reproduction. Postmating prefertilization interactions between males and females (e.g., sperm competition, sperm storage by females) also play a central role in reproduction. Moreover, post-copulatory interactions may have broad evolutionary implications. For example, postmating sexual selection may contribute to the evolution of reproductive isolation. Most research on post-copulatory pre-fertilization biology in flies has focused on male-expressed proteins such as seminal fluid proteins. Though it is well known that female flies are not simply passive vessels and that female-mediated interactions with male products are important to reproductive success, little is known about the genetics or population genetics of female postcopulatory biology. The proposed research aims to begin filling this basic gap in our understanding of Drosophila females. The specific aims of the proposed work are to i) use molecular approaches to identify candidate genes potentially affecting female postcopulatory phenotypes in D. melanogaster, ii) use QTL analysis of recombinant inbred lines of D. melanogaster to identify regions of the genome harboring natural variants affecting female post-copulatory phenotypes, iii) use molecular population genetics to test hypotheses on the causes of variation in candidate genes, iv) investigate divergence of female postcopulatory phenotypes and evolution of postcopulatory incompatibilities in the melanogaster group.