The integration of craniofacial elements during development in mammals is studiedin a phylogenetic approach. Developmental series of four metatherian (marsupial) and five Eutherian (placental) taxa are examined, and the sequence of emergence of major characters of the central nervous system, the cranial skeleton and the cranial musculature is reconstructed. These sequences are transformed into a series of characters that are then mapped onto the phylogeny of the taxa. This phylogenetic approach makes it possible to distinguish between characters thatare uniform across all mammals, and those that differentiate marsupials and placentals. The most significant difference between the two groups involves the relative timing of the developmentof the central nervous system and the somatic system. The central nervous system differentiates far in advance of the bones and muscles of the head in eutherians. In metatherians, somatic elements, particularly of the face, are accelerated. Additional other differences between the two groups of mammals are discussed. Many elements, however, are unchanged, and develop in a consistent relation despite overall shifts in development. These data are used to test several hypotheses on the ways that cranial development in mammals is integrated.