Tandemly repeated (dC-dA)(n) (dG-dT)(n) sequences occur abundantly and are found in most eukaryotic genomes. To investigate the level of conservation of these repeat sequences in nonhuman primates, we have analyzed seven human chromosome 13 dinucleotide (CA)(n) repeat loci in chimpanzees by DNA amplification using primers designed for analysis of human loci. Comparable levels of polymorphism at these loci in the two species, revealed by the number of alleles, heterozygosity, and allele sizes, suggest that the (CA)(n) repeat arrays and their genomic locations are highly conserved. Even though the proportion of shared alleles between the two species varies enormously and the modal alleles are not the same, allelic lengths at each locus in the chimpanzees are detected within the bounds of the allele size range observed in humans. A similar observation has been noted in a limited number of gorillas and orangutans. Using a new measure of genetic distance that takes into account the size of alleles, we have compared the genetic distance between humans and chimpanzees. The genetic distance between these two species was found to be ninefold smaller than expected assuming there is no selection or mutational bias toward retention of (CA)(n) repeat arrays. These findings suggest a functional significance for these microsatellite loci.