A total of 1,700 host-seeking Simulium ochraceum were allowed to feed to repletion on human volunteers heavily infected with Onchocerca volvulus and then were marked and released. Seventy-three (4.3%) of the flies were recaptured. The gonotrophic cycle of S. ochraceum, including the host seeking and oviposition phases, was approximately 74-97 hr in duration; however, for a few flies the cycle was up to an additional 24 hr in length. The frequency distribution of recaptured flies indicated that 2 gonotrophic cycles may be completed in 7-8 days. The large proportion of flies recaptured with sac-stage follicular dilatations indicates that S. ochraceum may begin to search for and can rapidly locate a new host almost immediately after oviposition. Larvae of O. volvulus were found in 15 (20.6%) of the recaptured flies. Early first stage larvae were present in host-seeking flies that had completed 1 gonotrophic cycle: 12 of 13 infected flies recaptured between 74 and 97 hr had early first stage larvae. Development of O. volvulus larvae to the infective stage was estimated to require approximately 11 days at the study site. During this time, S. ochraceum would normally have completed 3 gonotrophic cycles and be in the host seeking phase of a fourth at approximately 11 days after ingesting microfilariae.