A technique was developed for estimating the relative age of field-collected females of the Simulium damnosum complex. Females were allowed the opportunity to blood-feed while attacking and held 48 hours to permit partial maturation of the ovaries. The ovaries were then dissected out and the number of maturing oocytes was counted. Nullipars were easily identified by their lack of follicular relics. The remaining parous females generally fell into one of four groups depending on their egg counts. Wihout a laboratory colony for specific reference it is presently impossible to confirm that the fecundity groups represent successive gonotrophic cycles. Nevertheless, there is little doubt that the data demonstrate a decreasing level of fecundity in flies of advancing age thus allowing direct comparisons to be made between different populations. The epidemiological significance of this is discussed.