ABSTRACT.Christophers' stages of ovarian development are elaborated and redefined to provide a more useful descriptive tool for mosquito physiology. Vital staining is advocated to help identify the earliest stages of vitellogenic follicles. By use of vital staining it has been confirmed that in Aedes aegypti (L.) early vitellogenic ovaries contain negligible numbers of dormant follicles, and that fecundity is correlated with food intake by regulated resorption of developing follicles. The concept of developmental gates in mosquito oogenesis is discussed.