In the fall, adult face flies enter a facultative diapause in which mating and ovarian maturation do not take place, and their fat bodies become hypertrophied. Laboratory studies implicate short photoperiod as the primary inducer of diapause, but the response is temperature-dependent. Among field populations in Ames, Iowa, however, most teneral adults begin to enter diapause in the second week of September every year with no detectable effect of temperature. A range of light intensities<1,730 lux (160 ft-c) was applied to populations of flies at different photoperiods and temperatures. Precise calibration of light intensity was achieved with crossed polarizing filters placed over populations of eclosing flies. In a split-plot experiment, reduced light intensities led to significantly more diapause in each of four photoperiods. There was no interaction between photoperiod and light intensity. Effects of photoperiod and light intensity on diapause were obtained in split but not constant temperatures.