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To understand the origin and past dissemination of human T-cell leukemia/lymphotropic virus type I (HTLV-I) in Latin America, we conducted a phylogenetic study of five new HTLV-I isolates from Argentina. We sequenced partial fragments of long terminal repeats (LTR) of the new HTLV-Is, and then the sequences were subjected to a phylogenetic analysis for comparison with other HTLV-Is of various geographical origins. Our results indicated that all the isolates were members of the Cosmopolitan group. Furthermore, most (four out of five isolates) of the new HTLV-Is belonged to the Transcontinental (A) subgroup, the most widespread subgroup of the four subgroups in the Cosmopolitan group. In this subgroup, they were closely related to HTLV-Is found in other South American countries including those of Amerindians, and were different from those found in Africa. In contrast, the remaining one HTLV-I (ARGMF) did not show any clear similarity to known HTLV-I isolates belonging to the Cosmopolitan group. The close similarity of South American HTLV-Is strongly suggests a common origin of the virus in this continent. Our results do not support the proposed idea of recent introduction of HTLV-I into South America as a consequence of the slave trade from Africa, where phylogenetically different HTLV-Is predominate.