Dimeric short tandem repeats are a source of highly polymorphic markers in the mammalian genome. Genetic variation at these hypervariable loci is extensively used for linkage analysis, for the identification of individuals, and may be useful for interpopulation and interspecies studies. In this paper, we analyze the variability and the sequences of a segment including three microsatellites, first described in man, in several species of primates (chimpanzee, orangutan, gibbon, and macaque) using the heterologous primers (man primers). This region is located on the human chromosome 6p, near the tumor necrosis factor genes, in the major histocompatibility complex. The fact that these primers work in all species studied indicates that they are conserved throughout the different lineages of the two superfamilies, the Hominoidea and the Cercopithecidea, represented by the macaques. However, the intervening sequence displays intraspecific and interspecific variability. The sites of base substitutions and the insertion/deletion events are not evenly distributed within this region. The data suggest that it is necessary to have a minimal number of repeats to increase the rate of mutation sufficiently to allow the development of polymorphism. In some species, the microsatellites present single base variations which reduce the number of contiguous repeats, thus apparently slowing the rate of additional slippage events. Species with such variations or a low number of repeats are monomorphic. These microsatellite sequences are informative in the comparison of closely related species and reflect the phylogeny of the Old World monkeys, apes, and man.