Seven strains of West nile virus isolated in Nigeria were investigated for their ability to form plaques in monkey kidney cell monolayers. Five strains antigenically related to one another produced plaques of about the same size 3 to 4 days after the addition of the overlay medium. The two other strains closely related to each other produced no plaques. Their inability to produce plaques was regarded as a significant characteristic of the intratypic group to which the two strains belong.