The host range for hepatitis A virus (HAV) is limited to man and several species of non-human primates, and involvement of vertebrates other than primates in HAV circulation is unlikely. Spontaneous hepatitis A infection has been reported to occur in captive non-human primates including the great apes (chimpanzee) as well as Old World (cynomolgus, African vervet, stump-tailed) and New World (aotus) monkeys. The presence of anti-HAV antibody in the sera of newly captured monkeys of these species shows that infection may also spread in their natural habitat. HAVs isolated from spontaneously infected monkeys, although antigenically closely related to human HAV, exhibit a significant degree of genomic heterogeneity. There are at least four distinct simian HAVs differing from each other and from all human HAV strains. It is suggested that each virus is native to a given species reflecting evolutionary relationships among HAVs and their hosts in the order of Primates.