Standard curves for the regression of pteridine fluorescence in the head upon chronological age were constructed for laboratory-reared Glossina adults. Of the five species investigated, the technique was a reliable predictor of age in G. morsitans morsitans Westwood, G. pallidipes Austen, G. palpalis palpalis (Robineau-Desvoidy) and G. tachinoides Westwood, but was not reliable for G. austeni Newstead. Field-caught tenerals of both sexes of G. p. palpalis, field-caught teneral females of G. pallidipes and tenerals of both sexes of G. m. morsitans released on an island in Lake Kariba, Zimbabwe, had significantly higher pteridine fluorescence levels than tenerals hatched in the laboratory. However, the regression of fluorescence on age produced similar slopes for both laboratory-reared and field-caught flies. From this, it is concluded that estimates of the ages of field-caught flies can be corrected by simply subtracting the difference in fluorescence values between field-caught and laboratory-reared tenerals. In practice, this amounts to shifting the frequency distribution of different age categories to bring the youngest age group back to day zero. A comparison of the fluorescence technique and the ovarian dissection method of determining the ages of females showed that over a wide range of ages the former was at least as accurate as the latter and should be the technique of choice for most field investigations.