Thirty-four strains of Congo virus, an agent previously unrecognized in West Africa, were recovered from vertebrate and arthropod material collected in Nigeria during arbovirus investigations in the period July 1964 through December 1968. All isolations were made in infant mice. Five of the strains came from the blood of market animals; one from pooled liver and spleen of the hedgehog. Atelerix albiventris; 27 from ticks (four species of Hyalomma, Amblyomma variegatum, and Boophilus decoloratus); and one from Culicoides spp. The behavior of the strains in infant mice is described. A female Hyalomma rufipes that had engorged on an experimentally infected calf was found positive for Congo virus 18 days after dropping and 24 days after viremia was last detected in the calf.