Field populations of face flies, Musca autumnalis DeGeer (Diptera: Muscidae) were sampled serially in the late summers and autumns of 1985–96, in order to measure the dynamics of diapause development in Iowa, U.S.A. Fat body hypertrophy and ovarian development were assessed through dissection, and pteridine quantities in the fly heads were measured spectrofluorometrically to estimate their physiological and chronological ages. Published laboratory studies of diapause induction established the expectation that diapause incidence would vary over successive years with prevailing temperatures. The expectation was not met by field data. Patterns in the frequencies of hypertrophied fat body and of vitellogenic nullipars indicated that about 50% of recruited adults began developing into diapause in the second week of September in each year. Degree‐day summations for each year from 1985 to 1996 suggested that the hypothetical mean optimal time for switching into diapause would be in the second week of September, because a rapidly diminishing proportion of eggs laid later would result in viable, overwintering flies. It was concluded that face flies do not use daily temperatures in late summer, but instead cue on photoperiod for diapause.