Postweaning development of the skull of Didelphis albiventris was studied in juveniles (3.5–8 months), subadults (7.5–9.5 months), and adults (>9.5 months). Analysis of allometry of 15 skull measurements was performed on a continuous growth series of 52–61 specimens to evaluate quantitative ontogenetic changes. Complex modifications occurred in cranial bones or regions such as the palate, processes for origin and attachment of jaw muscles, mandibular joint, frontal region, braincase, occipital bones, and petrosal. All measurements except zygomatic breadth were either positively or negatively allometric. Neurocranial components grew with strongly negative (<1) coefficients of allometry. Adult proportions of the masticatory apparatus arose from the counteraction of developmental trends; for example, the space for large temporal muscles was provided by isometric growth of the zygomatic breadth versus slower growth of the braincase. We interpret most of the postweaning developments as a function of the shift from milk suckling to active mastication.