The pterin age-grading method was applied to natural populations of stable flies sampled for 3 years from diverse locations in Minnesota and Iowa. Significant differences were detected among years in mean degree-day ages per fly. Fly ages, in degree-day units, were exponentially distributed. A two-parameter Weibull distribution closely fit the fly survival distributions. Mean expectations of life did not differ significantly between males and females and were estimated to be 86.6 degree-days above a 6.5°C threshold. Conventional age-grading techniques based on ovarian morphology applied to Iowa females showed that large fractions of the populations experienced delays in vitellogenesis. The reproductive rate, in terms of lifetime ovipositions, was less than the maximum sustainable rate suggested by ambient temperatures.