In the house fly, oogenesis from stages 1 to 4 occupied the last 2 days of the pupal period. The completion of the 1st ovarian cycle from stages 4 to 14, in the well-fed female at 25[degree]C, required between 3 1/2 and 4 days. Growth of the follicle approached a constant exponential rate from stages 1 to 10, The period of vitellogenesis was divided into 2 phases an early phase from stages 8 to 10, and a late phase of slower growth from stages 11 to 12 after the vitelline membrane was formed. Reproduction in the adequately fed female showed ovarian synchrony, all oocytes of a cycle developing at the same rate. In females fed only sugar water, oogenesis was arrested at stages 7 to 8, the potential resting phase. After oviposition, subsequent cycles from stages 7 or 8 were completed in about 2 days. As long as a mature egg was retained in the ovariole, the follicle of the next cycle remained at stage 8. The dynamics of the growing oocyte was related to changes in the nurse cells and follicular cells. The nature of the yolk spheres at different stages of development and the formation of the egg membranes was investigated.