Employing neutralization tests, eight Nigerian strains of West Nile virus were investigated to determine antigenic relationships among them. One virus strain differed so significantly from the others and was later found to be a strain of Usutu virus. The other seven strains were differentiated into two serological intratypic groups dependent on their cross reactions with two strains which served as prototypes for each group. It was found that for epidemiological studies, screening of sera and sero-diagnosis, strain 7019 which had a broad spectrum of reactivity with other strains was a most suitable strain for detecting and evaluating antibodies to West Nile virus.