A strain of dengue type 3 virus was isolated from the blood of an Antiguan patient in cell cultures of African green-monkey kidney during studies on a dengue outbreak that occurred in the Caribbean region in 1963 and 1964. The virus was adapted to mice by blind intracerebral passage. Illness appeared in mice at the 11th mouse-brain passage. Four other agents, presumed to be dengue type 3 strains, were isolated from Antiguan patients. Serologic studies on paired serum samples from patients of five Caribbean territories showed that the outbreak was due to a group B arbovirus.