The antigenic relationships of 21 known or presumed Phlebotomus fever group serotypes and of 2 ungrouped, solvent sensitive, sandfly-associated arboviruses (Pacui and Charleville) were studied by complement fixation, plaque neutralization, and hemagglutination-inhibition methods. Results of complement fixation and neutralization tests were specific, allowing clear separation of the various serotypes, while those of the hemagglutination-inhibition test showed broader crossing and lack of specificity. Pacui virus was shown to be a member of the Phlebotomus fever serogroup. Six new Phlebotomus fever group serotypes are also described, increasing the known members of the group to 22. The implications of these and other recent data about the epidemiology of sandfly fever are discussed.