An investigation of an extensive outbreak of febrile illness during the months of April, May, and June 1965, in the city of Nagpur, Maharashtra State, showed that the main etiological agent was chikungunya virus. Dengue type 4 and Chandipura viruses were also active during this period. In all, 26 strains of virus were isolated from 60 acute phase human sera, and of these strains, 23 were identified as chikungunya virus, 2 as Chandipura, and 1 as dengue type 4. Five strains of chikungunya virus and 9 strains of dengue type 4 virus were isolated from 34 pools of Aedes aegypti collected from the affected areas. Results of complement fixation tests with acute-convalescent paired serum samples and single convalescent sera confirmed that chikungunya virus was the main etiological agent. The significance of these findings is discussed.