Five distinct, hitherto undescribed arthropod-borne viruses have been characterized from among more than 70 strains isolated from man, sentinel animals and mosquitoes in the forested area around Belém, Pará, Brazil. These viruses have been designated by Causey et al. Marituba, Murutucu, Apeu, Caraparu and Oriboca. The viruses are immunologically related among themselves and have failed to show any antigenic overlap with other groups of arbor viruses; they therefore constitute a new antigenic group to which the name, group C, has been given. Within this group, by HI and NT tests, Marituba and Murutucu viruses were found to be closely allied, as were Apeu and Caraparu. Each pair, however, was easily separable from the other and from Oriboca virus. By CF test, all viruses showed pronounced overlap, with the exception of two strains of Caraparu virus which were sharply separable from the other viruses.