A female pigtailed macaque ("Macaca nemestrina") with unusual physical characteristics, deficits in learning and cognitive tasks, abnormal social behavior, and abnormal reflexes and motor control was followed from birth until 3 years of age and found to have trisomy 16, which is homologous to trisomy 13 in humans. The animal described here showed similar features to cases of trisomy 16 and 18 (human trisomy 13 and 18, respectively) reported previously in nonhuman primates. However, both significant differences and similarities were found when compared with the homologous human trisomy. Evaluation of the genetic components of these disorders as well as systematic developmental evaluation can lead to new insights into the genetic basis of speciation, development, and the underlying differences between humans and their closest living relatives.