Abstract X-ray-induced mitotic recombination was used to follow the development and function of the female germ line in Drosophila melanogaster. Clones marked by maternal effect mutations which alter the morphology of the egg [fs(1)K10] or the phenotype of the resulting progeny (maroonlike) were produced in trans-heterozygotes irradiated during embryonic, larval, or pupal development or as 5-day-old adults. Judging from the size of clones induced at the blastoderm stage, only five to ten of the pole cells observed on the surface of the embryo contribute to the germ line. Most of the K10 clones induced during embryonic and larval development were associated with mal twin spots, indicating that both daughters of the irradiated germ cell remained in the germ line and gave rise to eggs in the adult. During larval life the number of cells increases logarithmically and reaches a maximum of 110 at 24 hr after pupation. The same value was obtained for 5-day-old adults. In contrast to the mosaic females produced as embryos and larvae, mosaics obtained after pupal and adult irradiations were of two types, those laying only one K10 egg and those laying several K10 eggs distributed over the lifespan of the adult. This result indicates that the stem cell divisions characteristic of the adult period have begun shortly after pupation. About 9 to 11 days are required for an irradiated stem cell to produce its first clonal K10 egg, and two-thirds of this time is spent in the germarium. Each ovariole possesses on the average two to three functioning stem cells. This multiplicity of stem cells was confirmed by the recovery of mosaic ovarioles when mal heterozygotes irradiated as adults or late larvae were stained for aldehyde oxidase activity.