Despite strong evidence to the contrary1, 2, 3, 4, 5, speculation continues that the AIDS virus, human immunodeficiency virus type 1 (HIV-1), may have crossed into humans as a result of contamination of the oral polio vaccine (OPV)6, 7, 8. This ‘OPV/AIDS theory’ claims that chimpanzees from the vicinity of Stanleyville — now Kisangani in the Democratic Republic of Congo — were the source of a simian immunodeficiency virus (SIVcpz) that was transmitted to humans when chimpanzee tissues were allegedly used in the preparation of OPV6, 7. Here we show that SIVcpz is indeed endemic in wild chimpanzees of this region but that the circulating virus is phylogenetically distinct from all strains of HIV-1, providing direct evidence that these chimpanzees were not the source of the human AIDS pandemic.