The existence of onchocerciasis in Ethiopia, especially in its South-Western Region was known. The naturally infected Simulium has not been reported in the region, though there has been reported the existence of S. danznosum, S. woodi etc. which were identified as the vectors of this disease in other parts of Africa. The authors obtained a number of Onchocerca volvulus from S. damnosum collected by biting-catch method in the field in August and November, 1971. Dissection was made on the 975 Simulium out of about 1000 caught at the riverside of Gojeb and those caught at Didessa riverside. The infection rate ranged from 10.2 to 12.9% in the former and 20.0 to 40.6 in the latter. The sausage type was most frequently found and the late stage type was detected in 11 % of Simulium. An evidence was given that S. damnosum is the main vector of onchocerciasis in Ethiopia, since this species formed an absolute majority of the flies collected by biting catch and no filaria was detected in the other species.