There is considerable genetic diversity among viruses of different subtypes (designated A to J) in the major group of human immunodeficiency virus type 1 (HIV-1), the form of HIV that is dominant in the global epidemic(1-3). If available, HIV-1 sequences pre-dating the recognition of AIDS could be crucial in defining the time of origin and the subsequent evolution of these viruses in humans. The oldest known case of HIV-1 infection was reported to be that of a sailor from Manchester who died of an AIDS-like illness in 1959 (refs 4-6); however, the authenticity of this case has not been confirmed(7,8). Genetic analysis of sequences from clinical materials obtained from 1971 to 1976 from members of a Norwegian family infected earlier than 1971 showed that they carried viruses of the HIV-1 outlier group(9,10), a variant form that is mainly restricted to West Africa(1). Here we report the amplification and characterization of viral sequences from a 1959 African plasma sample that was previously found to be HIV-1 seropositive(11). Multiple phylogenetic analyses not only authenticate this case as the oldest known HIV-1 infection, but also place its viral sequence near the ancestral node of subtypes B and D in the major group, indicating that these HIV-1 subtypes, and perhaps all major-group viruses, may have evolved from a single introduction into the African population not long before 1959.