African monkeys can be naturally infected with SIV but do not progress to AIDS. Since mutations in the human CCR5 gene have been shown to influence susceptibility to HIV infection and disease progression, we have now investigated whether mutations in CCR5-coding sequences in African nonhuman primates can explain species-specific differences in susceptibility to lentiviral infection. The animals studied comprise chronically infected monkeys corresponding to four natural hosts of SIV (Cercopithecus aethiops, Cercopithecus pygerythrus, Cercopithecus sabaeus, and Cercopithecus tantalus), noninfected animals from three species that are known to be susceptible to SIV infection (Cercopithecus patas, Cercopithecus Ihoesti, and Pan troglodytes), and monkeys of six species that do not carry SIV in the wild (Cercocebus galeritus, Cercocebus aterrimus, Cercopithecus ascanius, Cercopithecus nictitans, Cercopithecus neglectus, and Cercopithecus cephus). We observed a high degree of genetic divergence among the species. The rate of accumulation of amino acid mutations was, however, not higher in SIV carriers than in other nonhuman primates. No homozygous premature stop codons, deletions, or frameshift mutations were detected. In at least two animals, one infected AGM (Cercopithecus tantalus) and one noninfected monkey (Cercocebus aterrimus), the CCR5 alleles identified encode functional proteins, as they were identical in terms of amino acid sequence to that of functional CCR5 reported in the literature. We found no other consistent differences in the genetic variability of CCR5-coding sequences between the nonhuman primates that are carriers of SIV and those that are not.