Antigenic relationships among 24 bunyaviruses of the Simbu serogroup were determined by complement-fixation (CF), serum dilution-plaque reduction neutralization (N) and, where possible, hemagglutination-inhibition (HI) tests. By CF, three distinct complexes of closely related viruses were identified within the serogroup. Nola and Thimiri viruses, which showed little relationship with other members of the serogroup, may represent two additional complexes. N tests in Vero cells showed that individual viruses generally were distinguishable with little difficulty. Aino and Kaikalur viruses were indistinguishable by CF or N. Seven viruses showed hemagglutination activity, and antigenic relationships among these viruses by HI paralleled those established by N tests. A classification scheme, based on both CF and N test results, for the viruses of the Simbu serogroup is proposed.