With the exception of the work of Schultz (1960), cranial growth in Ateles is not well documented. This paper describes the results of a detailed quantitative study of cranial ontogeny in male and female Ateles geoffroyi. Using Euclidean Distance Matrix Analysis (EDMA), local areas of form change due to growth within spider monkey crania are identified. We found substantial change local to the zygomatic region in the face, identified mediolaterally directed changes in the palate, detected relatively larger amounts of change local to the anterior neurocranium compared to the posterior neurocranium, and demonstrate a greater amount of basicranial growth along a mediolateral axis than previously reported. Cranial sexual dimorphism is also examined. A. geoffroyi is noted for being monomorphic, and we found a general similarity between male and female cranial forms at all developmental ages. However, differences in overall cranial size between the sexes were found in the oldest subadult age group but not between male and female adults. This difference suggests that A. geoffroyi females attain their adult cranial form slightly before males and implies a pattern of earlier onset of female maturity relative to males. © 1993 Wiley-Liss, Inc.