The prevalence of human T-lymphotropic virus (HTLV) type-1 antibodies was determined in the bonnet monkeys, living naturally, within about 30 km radius of Vellore (south India). Sera from 157 animals, collected between January 1982 and May 1993 were screened for the presence of HTLV-I infection by a particle agglutination test (PAT). When sera repeatedly reactive in PAT were subjected to indirect immunofluorescence and western blot tests, 63 (40%) were confirmed to be positive for HTLV-1 antibody. These findings are significant in the light of recent reports that HTLV infection is endemic to southern India.