Cranial ontogenetic analyses improve our understanding of function from developmental, ecological, and evolutionary perspectives. Akodon montensis is an abundant, omnivorous forest species that occupies many habitat types. We used traditional and geometric morphometric approaches to describe ontogenetic variation in skulls of A. montensis. We tested for sexual dimorphism and described patterns of variation associated with both age and size based on 6 postweaning age classes. We found no evidence for sexual dimorphism. Growth patterns showed an initial narrowing of the braincase, and associated changes in the rostrum as specimens reached adulthood. Older animals had an elongated rostrum and palate. Geometric morphometric analysis revealed allometric variation associated with the basicranium for the entire age series, while traditional morphometric analyses showed allometric variation in the facial component. The patterns found for A. montensis are similar to those of other species of Akodon. We characterize ontogenetic patterns for Akodontines, the second most diverse Sigmodontine tribe, and a model group for studies of shape change in generalist rodents.