According to the 'developmental constraint hypothesis' of comparative mammalian neuroanatomy, brain structures enlarge predictably as the entire brain grows both ontogenetically and phylogenetically. In this study, brain and cerebellum volumes are measured from in vivo magnetic resonance scans of 44 primates from 11 haplorhine species. After controlling for overall brain volume, the cerebellum in both pongid and hylobatid apes is, on average, 45% larger than in monkeys. These results demonstrate that all primate brains are not similarly organized and that developmental constraints are not tight enough to preclude selection for increased cerebellar volume independent of selection on overall brain size.