Spontaneous ectopic uterine ovarian tissue was found in 9 of 118 cynomolgus monkeys (Macaca fascicularis), an incidence of 7.6%. These ectopic tissues were minute and were incidentally observed in the parametrium by microscopic examination. They were composed of primordial or primary follicles and ovarian stroma. Ectopic ovarian tissue may result from disturbed migration of the ovarian primordium during embryogenesis.