One hundred and twenty-nine free-ranging rhesus monkeys on the island of Cayo Santiago were surveyed for the nematode Anatrichosoma cutaneum. The purpose of the study was to determine the prevalence of the nasal and cutaneous forms of this parasite in untreated rhesus monkeys maintained in a seminatural subtropical environment. The average prevalence of the nasal form in the sampled Cayo Santiago macaques was 23.2%. Adults were more commoly effected than yearling monkeys with 31.2% and 12.8% parasitized, respectively. Newborn infants were negative. No cases of the cutaneous form were identified. This study determined that nasal Anatrichosomiasis is common in the Cayo Santiago macaques. Further studies are necessary to establish the roles of intermediate host(s) and rhesus monkey behavior in the life cycle of this parasite in order to determine how Anatrichosomiasis has persisted for over 40 years in this colony.