Pterins accumulate in the head capsules of horn flies, Haematobia irritans irritans (L.), as a linear function of time and temperature. Pterin concentrations were used to estimate chronological ages and to establish correlations between chronological age and ovarian development and reproductive success in 12 horn fly populations in 1988 and 1989. Male ages were estimated spectrofluorometrically. There were statistically significant differences between years in population age structure measured by pterins. Survival rates estimated from pterin concentration distributions were consistent with a one-parameter exponential model with constant survival rate. Mean daily survival rates were 0.81 for females and 0.84 for males in 1988 and 0.66 and 0.75 in 1989. Mean lifetime egg production was approximately 26 eggs per female in 1988 and 8 in 1989. Female reproductive success was close to the maximum possible, i.e., there were no net delays in oviposition. Analysis of gonotrophic age distributions provided survival estimates that suggested an increasing risk of mortality with age or age-related biases in sampling.