Abstract Cortical sulci were digitized and their lenghts determined with 3-dimensional computer technology on 335 endocranial casts from rhesus monkeys with known maternal genealogies, ages at death, and sex. Non-metric data were also collected from 403 endocasts. Frontal lobes were directionally asymmetrical with lengths of the left central, right rectus (principal), and right lateral orbital sulci significantly longer. The positions of the medial and lateral ends of the central sulcus were significantly different in the two hemispheres, and there was significant protrusion of the frontal lobe (petalia) on the right side. Together, these data indicate elongation of the right orbital and dorsolateral frontal lobe. The asymmetries reported here probably involve short-term memory for visual information. This raises interesting questions about the extent to which macaques are right hemisphere dominant for processing visual information as compared to humans.