Sera collected from 1,102 individuals in 14 populations of the southwestern Pacific between 1956 and 1979 were tested by ELISA for antibodies to human T-cell leukemia virus type I (HTLV-I). Selected sera were also tested by particle agglutination and immunoblotting. Six of the populations had prevalences of antibodies greater than 4%, two populations had prevalences greater than 15%. Six populations had antibody prevalences of 2% or less. Three populations from the coast and northern islands of New Guinea had high prevalences of antibodies, while three New Guinea highland groups had virtually none. One population from the Solomon Islands had a high prevalence, while two others had very low prevalences. Two populations from small remote islands in Vanuatu both had high prevalences. Pacific sera did not neutralize a standard strain of virus readily neutralized by Japanese, European, and American sera. We conclude that infections with HTLV-I, some acquired more than 20 years ago, are widespread throughout the southwestern Pacific, even in several very isolated populations, although others have been spared. Some strains of HTLV-I in populations of the Pacific may have substantially different envelope proteins from prototype strains of America, Europe, and Japan.