Three hitherto undescribed arthropod-borne viruses, two from Brazil and one from Trinidad, have been found to be immunologically related to one another and, thus, to comprise a new immunological group. The prototypes of these viruses are: Be An 277 to which Causey et al. have given the name Guamá virus; Be H 151 named by Causey et al. Catú virus; and Tr 8362 named Bimiti virus by Spence et al. It is proposed that the group be given the name Guamá.