Bertioga and Anhembi are two newly recognized arboviruses that were isolated in Saõ Paulo, Brazil. Bertioga was isolated from sentinel mice exposed in a forest of that name, and was shown to belong serologically to the Guama group of arboviruses. Anhembi, the second virus described, was isolated from pools of mosquitoes (Phoniomyia pilicauda and trichoprosopon pallidiventer), and from a spiny rat (Proechimys iheringi), and was shown to be a new member of the Bunyamwera group. The viruses are sensitive to sodium desoxycholate and are filterable through a millipore membrane of 450 mum pore size. Both are pathogenic for suckling mice and grow well in Vero cells, producing clear cytopathic effects. Detailed antigenic comparisons of Anhembi virus with other members of the Bunyamwera group isolated in the Americas, and of Bertioga virus with the members of the Guama group are presented. Both are believed to be new arboviruses in their groups.