Two strains of eastern encephalitis (EE) virus were isolated in the Amazon region of Peru near Pucallpa, Loreto Department, using sentinel hamsters. EE virus antibodies were found in healthy horses at both Pucallpa and Iquitos in the same Department. Fourteen group C and four Guama group arboviruses were recovered from sentenel hamsters and mosquitoes near Iquitos. The group C agents were Caraparu-Ossa, Marituba, and Oriboca-Itaqui viruses, and the Guama group agents were Bimiti virus. Besides providing a detailed account of these investigations, this article includes a current list of known arboviruses of the American tropics that can be detected with sentinel hamsters.