Some poorly recognized features of onchocerciasis were defined in the Republic of Chad in Central Africa in two hitherto unrecognized holoendemic foci. One of these was Ouli Bangala, situated at 7° 50′N latitude and 15° 50′E longitude, the other Masidjango at 10° 10′N latitude and 19° 25′E longitude, on the southern shore of Lake Iro near Boum Khebir. A study of the village of Ouli Bangala (population, 398) revealed the following: the recovery of microfilariae of Onchocerca volvulus in urine specimens from an unexpectedly high percentage of the population (11%); the diagnostic value of routine chest roentgenograms in onchocerciasis; the suppression of delayed hypersensitivity reactions as observed in routine tuberculin skin tests in persons with onchocercoma or skin atrophy or both; and the finding that patients with onchocercal microfilaruria generally have lower serum antibody titers to indirect hemagglutination tests with Dirofilaria immitis. The finding of a high rate of onchocercal microfilaruria was attributed both to the technique employed, that of triple concentration of complete urine specimens (settling overnight plus two centrifugations) and to the severity of infection. The presence of microfilariae in urine samples was closely associated with the intensity of skin infection, with a higher risk of exposure to bites of Simulium damnosum, and with the presence of other well-known manifestations of onchocerciasis.