THE ancestors of the human immunodeficiency viruses (HIV-1 and HIV-2) may have evolved from a reservoir of African non-human primate lentiviruses, termed simian immunodeficiency viruses (SIV)¹. None of the SIV strains characterized so far are closely related to HIV-1^2–6. HIV-2, however, is closely related to SIV (SIV_mac) isolated from captive rhesus macaques (Macaca mulatta)⁷. SIV infection of feral Asian macaques has not been demonstrated by serological surveys^8,9. Thus, macaques may have acquired SIV in captivity by cross-species transmission from an SIV-infected African primate. Sooty mangabeys (Cercocebm atys), an African primate species indigenous to West Africa, however, are infected with SIV (SIV_sm) both in captivity^9–11and in the wild (P. Fultz, personal communication). We have molecularly cloned and sequenced SIV_sm and report here that it is closely related to SIV_mac and HIV-2. These results suggest that SIVsm has infected macaques in captivity and humans in West Africa and evolved as SIV_mac and HIV-2, respectively.