Bones of the face and cranial vault meet at sutural boundaries. These sutures are of great importance for craniofacial growth. Although the effects that the sutures have on modulating craniofacial strains have been investigated, how sutural fusion influences primate craniofacial biomechanics and adaptation are less considered. Confounding this problem is the lack of any systematic data on patterns of craniofacial sutural fusion from animals of known age and sex. This study examined the status of 28 sutures in Macaca mulatta skulls from a collection of animals of known age and sex from Cayo Santiago, Puerto Rico. Survival analysis showed that most animals died before all sutures fused. There was high variation in the age at which individual sutures or sutural sections were fused in M. mulatta, and significant differences in the amount of sutural fusion among regions and between males and females. Intensive fusion of sutures took place between ages 5 and 15. Sutures in the facial area tended to be less fused than in the cranial vault. Between adolescence and adulthood, males tended to have more sutural fusion than females, especially in the facial area. These differences might be biomechanical adaptations during ontogeny to craniofacial sexual dimorphism. These findings enrich our understanding of variation in sutural morphology in rhesus monkeys. Comparative information across primate species is essential for understanding the biomechanics of craniofacial form throughout primate evolution. 2006 Wiley-Liss, Inc.