Oogenesis in Drosophila requires a signi¢cant amount of phosphorus. Oocytes mature in follicles, each of which contains 15 highly polyploid, transcriptionally active chromosomes. We show that the demand for phosphorus is met in part from the male's ejaculate following mating. Females incorporate phosphorus-32 from radiolabelled males into their ovaries, speci¢cally into their nucleic acids. Male-derived phosphorus is also present in signi¢cant amounts in mature oocytes. The mechanism by which phosphorus uptake from the female reproductive tract occurs must di¡er from that previously reported for radiolabelled carbon and hydrogen derived from ejaculatory proteins, as phosphorus uptake is observed in species not showing female incorporation of radiolabel derived from ejaculate proteins.