Diapause induction in female face flies,Musca autumnalisDeGeer, was studied in the laboratory. A visual scale for fat body quantity was developed and tested to estimate crude lipid content. Dissected female flies were defined as being in apparent diapause if they possessed previtellogenic ovaries and hypertrophied fat body at the time they were removed from a diapause-inducing environment. In contrast, flies were defined as committed to diapause only if they still possessed the same symptoms after 8 additional days in a warm, long-day environment. Availability of blood did not significantly affect apparent diapause induction. An experiment using 15 combinations of temperatures and photoperiods revealed that temperature had a strong effect on rates of ovarian growth, fat body growth, development of apparent diapause, and development of commitment to diapause. The latter three were fastest at ca. 23°C, well below the temperature of fastest ovarian growth. Short photoperiods had no effect on ovarian growth but did accelerate development into diapause. A conceptual model is presented that describes induction and development into diapause in female face flies as a competitive rate process between reproductive development and commitment to diapause.