Development of the testes and ovaries of the house fly, Musca domestica L., was studied from third-instar larvae to 6-day-old adults. Spermatogonial divisions occurred during the third-instar larvae and early pupae. Considerable meiotic activity took place during the pupal period, and sperm maturation was essentially completed by emergence. Oogonial divisions were observed during early pupation, with the first egg chambers being formed before emergence. Female imagos, at the onset of oviposition, possessed 1 mature egg and 2 immature egg chambers per polytrophic ovariole. Each egg chamber contained 15 nurse cells, with polyploid nuclei and 1 oocyte. The nurse-cell nuclei produced large amounts of RNA during vitellogenesis. Meiosis was not observed during oocyte maturation.