This study indicates that the California-encephalitis-virus (CEV) group is large and antigenically complex, as are several other arbovirus groups. The examination of many isolates reveals a wide spectrum of detectable antigenic variation. Many isolates appear to be essentially identical, especially those isolated at the same time in the same general area, but those isolated in more distant places, or in different years from the same area, are likely not to be the same. Even from the same area, two antigenically distinct types may be recovered at the same time (as observed in the Tampa Bay area of Florida in 1964). In the United States there appear to be at least eight types that, in our opinion, are distinguishable by the methods used. Data, principally complement-fixation, are presented, but supported in part by neutralization and hemagglutination-inhibition tests. The prototypes tentatively selected are CEV, snowshoehare, LaCrosse, trivittatus, Keystone, Jerry Slough, San Angelo, and Jamestown Canyon viruses. A group of isolates from near Toronto, Canada, were found to be closely related to the snowshoe-hare virus by cross-complement-fixation comparisons, while those sent from Panamá and the majority of those from Florida were found to be closely related to the trivittatus virus, and thus so designated. California and snowshoe-hare viruses cross-react with all, by both complement-fixation and neutralization tests, and can be used for identification of isolates or as diagnostic sera for grouping purposes. In addition, both form adequate hemagglutinating antigens, which many isolates do not readily form. Keystone virus might have some advantage as a CF reagent. Three exotic viruses of the group, Melao, Tahyna, and Lumbo, were included in the cross-comparisons. Relatively minor differences were found between Tahyna and Lumbo rather similar in degree to those found between snowshoe-hare and LaCrosse. If such differences are accepted as significant, the CEV group is currently composed of 11 types, nine being present in the Western hemisphere and two in the Eastern. This classification is presented as tentative, subject to confirmation by other methods and studies with other isolates.