Patterns of interspecific differentiation in saki monkey (Pithecia) skulls are quantitatively described. The taxonomic arrangement previously proposed by Hershkovitz ( Am. J. Primatol. 12:387–468) is consistent with quantitative differences in saki morphology. Discriminant analyses on 39 skull traits show that Pithecia species and subspecies are well-differentiated. Morphological distances (D2) among sakis clearly show the morphological unity of the pithecia-chrysocephala (Pithecia) and irrorata-vanzolinii-monacha (Monacha) species groups. The Pithecia species group is distributed north of the Amazon and has a smaller cranium than the Monacha group, distributed south of that river. Despite the size difference, multivariate static allometric patterns among sakis are quite similar. After removing size and allometric changes in shape from the data, species and subspecies are still differentiated, although to a lesser extent. D2 distances obtained from these scale-corrected data are similar in magnitude and pattern to the original D2, but show a closer similarity of P. monacha with the Pithecia group. P. monacha is a scaled-up version of the smaller sakis. Am J Phys Anthropol 124:000–000, 2004. © 2004 Wiley-Liss, Inc.