The antigenic, biological, and chemical properties of 24 selected Changuinola serogroup viruses were examined. The viruses tested were chloroform-resistant, and they were lethal to newborn hamsters after intracerebral inoculation. The prototype Changuinola virus strain (BT-436) replicated in mosquito and sandfly cell cultures. In complement-fixation tests, the viruses were broadly cross-reacting and indistinguishable; but by neutralization test at least 12 distinct serotypes were identified, and by PAGE of double-stranded RNA 22 distinct profiles were found. These data suggest that the Changuinola serogroup may be comprised of a large number of genetically different viruses. A brief review of the natural history of Changuinola serogroup viruses is also given.