Fragile X syndrome, the most common inherited form of mental retardation, arises in individuals with more than 200 CGG repeats in the 5′ untranslated region of the fragile X mental retardation 1 (FMR1) gene. Although CGG repeat numbers comparable to those found in the normal human population are found in various non-human primates, neither the within-species size variation nor the propensity for expansion of the CGG repeat has been described for any non-human primate species. The allele distribution has now been determined for FMR1 (homologue) CGG repeats of 265 unrelated founder females of Macaca mulatta monkeys. Among 530 X chromosomes, at least 26 distinct repeat lengths were identified, ranging from 16 to 54 CGG repeats. Of these alleles 79% have between 25 and 33 CGG repeats. Detailed examination of the CGG region revealed a conserved G (CGG)2 G interruption, although in no case was an AGG trinucleotide detected. Two animals carried borderline premutation alleles with 54 CGG repeats, within the region of marginal instability for humans. Thus, M. mulatta may be useful as an animal model for the study of fragile X syndrome.