1. In normal Cochliomyia hominivorax females, gross ovarian growth, correlated with the age of the adult from emergence to sexual maturity, was measured. Studies showed that the size of the ovary doubles between the first and second day of adult life, increases more than 5-fold between the second and third day, and exhibits a total increase of approximately 60-fold from emergence to sexual maturity. 2. A cytological study of the ovarioles was conducted to determine the sequence of events in normal oogenesis. The cytology of the reproductive system, from 5-day-old pupae to sexually mature females, is described. 3. The effects of gamma radiation on gross ovarian growth indicated that newly emerged females are more radiosensitive than 5-day-old pupae, and that irradiation of 2-day-old females has little effect on subsequent ovarian growth. 4. The cytopathology of the irradiated ovary was studied after similar doses of radiation were delivered to various developmental stages. The general sequence of events after treatment was as follows: After low doses, growth is slower than normal but not completely arrested. If treatment is given to 5-day-old pupae, grossly malformed oocytes are not often encountered, but second egg chambers frequently do not form. When females 0-4 hours old are irradiated, growth in the first egg chamber is delayed considerably, and is often followed by complete degeneration of the first egg follicle or the formation of grossly malformed oocytes. In these studies, the most radiosensitive stage encountered was that period during which the egg chambers contain nurse cells undergoing endomitotic replications of chromosomal material.