In the current study, we have assessed the distribution of black fly vectors and the occurrence of onchocerciasis among residents of the Firestone Rubber Plantation (FRP) in Liberia. The degree of endemicity of onchocerciasis on the FRP is correlated with the breeding of Simulium yahense on the Du River and Simulium soubrense on the Farmington River, as well as with the feeding behavior of these black fly vectors. S. yahense is highly anthropophilic and highly susceptible to Onchocerca volvulus, thus, it is apparently the primary vector of O. volvulus on the plantation. S. soubrense is predominantly zoophilic; consequently, it has a low capacity for transmitting of O. volvulus to humans. The prevalence of the disease is hyperendemic in the divisional camps near the Du River, affecting 77.6% to 94.4% of males and 64.9% to 88.9% of females. In the camps along the Farmington River, the disease is hypoendemic, affecting 26.1% to 49.2% of males and 25.6% to 35.1% of females. The mean intensity of infection in the camps near the Du River was 14.1 microfilariae (mf)/mg of skin in males and 13.7 mf/mg of skin in females. The density of microfilariae in people in the camps along the Farmington River was 4.8 mf/mg of skin in males and 1.9 mf/mg of skin in females. The age-specific infection in camps along both rivers substantially increased after the ages 21-30, reaching a maximum at ages 41-50. The mean density of mf/mg of skin in three anatomic locations (calf, hip, and shoulder) was 28.0, 24.5, and 14.1 mf/mg of skin, respectively, for those living on the Du River, and 10.2, 7.6, and 5.8 mf/mg of skin, respectively, for those on the Farmington River.