Six marking-recapture experiments were carried out with Simulium damnosum in the Cameroon Republic, five in the rain-forest and one in the Sudan-savanna zone. Adult flies were marked by applying a small spot of oil paint to the mesonotum while they were engorging with blood on the legs of volunteers infected with Onchocerca volvulus microfilariae. Catches of wild flies were carried out for 12-13 days after the first day of marking, and all marked flies recaptured were examined. Some of the flies recaptured on days 0, 1 and 2 were nulliparous and had probably been disturbed by the application of the paint before they had completed their blood-meal. The frequency distribution of recaptured marked parous flies returning for their "second" blood-meal rose to a peak early on day 4, but more flies returned earlier,three days after taking a blood-meal, than later on day 5. After day 5, the numbers of recaptured flies were too low to demonstrate any peaks corresponding to "third" and later blood-meals. The longest surviving fly was recaptured 10 days after marking. Twenty-six percent of the flies recaptured on days 3 and 4 contained developing O. volvulus larvae three-five days old, which had presumably been ingested as microfilariae during the blood-meal taken on day 0. Infective larvae first appeared in flies returning late on day 6, and the highest percentage of infective flies occurred on day 7. Infective larvae were found in recaptured flies until day 10, the last day on which marked flies were recovered. Of 929 flies marked on the Sudan-savanna experiment, three (0-32%) flies were recaptured. Two returned on day 4 and one on day 6.