Virgin adult female screw-worms, Cochliomyia hominivorax (Coquerel), were treated topically with an alkaylating agent, 2,5-bis(1-aziridinyl)-3,6-bis(methoxyethoxy)- p-benzoquinone, when they were either 0 to 4 hoursold (nurse cells in 1st egg chamber in process of endomitoticreplication) or 24±2 hours old (endomitotic replication completed). Observations and measurements were made of the size of the egg chambers, nurse-cell nuclei, and of the condition of the components of the ovarioles in 2-, 4-, and 7-day-old females. Treatment of young females had a much more drastic effect on egg development (produced complete infecundity) that was related to pathological conditions in the nurse-cell nuclei. Apparently treatment with an alkylating agent when the nurse-cell chromosomes are undergoing endomitotic replication completely disrupts this process and prevents the nucleus of these cells from attaining the proper degree of ploidy and the size required for normal vitellogenesis. Older females receiving the same treatment were significantly retarded in ovarian development but were eventually able to produce mature eggs and deposit a 1stegg mass, though development of eggs in the 2nd or 3rd eggs chambers was impaired.