Black fly vectors of onchocerciasis from three ecologically different Simuliid breeding habitats in the Firestone Rubber Plantation at Harbel, Liberia, were surveyed by human-biting collections conducted at weekly intervals over a 13-month period. Black flies were identified morphologically and the monthly and seasonal contribution of different vector species to the transmission of Onchocerca volvulus at each site was determined. Simulium yahense was identified as the predominant vector species at each site. Greatest populations of this species occurred during wet season (May-Oct), but its impact on transmission of onchocerciasis was most profound during dry season (Nov-Apr) when parity, infection, and infectivity were high. S. sanctipauli was the only other vector species captured, but biting populations of this species were small, and during wet season confined primarily to the vicinity of its breeding site in the Farmington River. Dry season populations of S. sanctipauli were also characterized by lower human-biting rates, and by higher rates of parity, infection, and infectivity. Monthly transmission potentials at each site were attributed primarily to S. yahense, with peak monthly transmission occurring during the dry season months of January-April. Against the WHO standard of 100 as a "tolerable" annual level of onchocerciasis transmission, annual transmission potentials for the three sampling sites were 94, 1,877, and 4,900, with highest values being calculated for S. yahense breeding sites.