Spectrofluorimetric methods were used to measure pterin quantities in the head capsules of adult face flies, Musca autumnalis De Geer. Quantities at eclosion were greater in males than in females, and were proportional to the head capsule width in each sex. Pterin accumulated as a linear function of time at constant temperature, at rates of 3.8 relative fluorescence units per degree-day in males and 1.68 units in females, both above a common base of 9.8 degrees C. A mark-release-recapture experiment revealed that pterin accumulated at a rate of 6.40 units per degree-day among males, twice as fast as in the laboratory. In contrast, accumulation among females was 1.35 units per degree-day, approximating their laboratory rate. Gonotrophic age grading of recaptured females indicated median development rates corresponded with expectations based on earlier laboratory studies, but substantial variation among females led to imprecise estimates of chronological age. Calibration curves for each sex are presented to estimate degree-day age from head capsule width and pterin content. The pterin age-grading method allows the field study of longevity of male face flies, and complements gonotrophic methods for the study of longevity and reproductive success of females.