Placement of viruses into serogroups was difficult, since not all the viruses were found to have hemagglutination activity. Viruses were considered ‘ ‘arthropod-borne viruses” based on circumstances of isolation, inactivation by sodium deoxycholate or ether, and other minimally descriptive methods. Intragroup relationships heretofore not detected by hemagglutination-inhibition were found; complement-fixation tests revealed finer distinctions within serogroups, and N tests were shown to be even more discriminating. Thirteen serogroups comprise the orbiviruses. Certain members of the bluetongue, epizootic hemorrhagic disease, and Eubenangee serogroups have been shown to share intergroup antigenic relationships. This indicates the existence of a “bluetongue supergroup”. The Palyam serogroup, which may share antigens with these viruses, could be included in a supergroup. Kemerovo serogroup viruses have been studied thoroughly, and assignment of individual viruses to antigenic complexes has been suggested. Although placed in what is known as “minor antigenic groups”, the viruses are not “minor” in terms of importance or interest, only in the size of the serogroups.