Abstract We analyzed ontogenetic patterns of landmarks for 169 laboratory-raised specimens of Calomys expulsus, at 0, 5, 10, 20, 30, 50, 100, 200, and 300 days of age, using two-dimensional geometric morphometrics. There is sexual dimorphism in size, with males smaller than females at earlier ages, but larger after 50 days. Differences in shape between sexes are strong only until 10 days of age, suggesting that shape is more constrained than size. Combining sexes, there is strong variation in size with age, reduced after 200 days, while most of the variation in shape occurs before 20 days. This dissociation is common for sigmodontine rodents, and might be the basis of heterochronic processes responsible for the morphological variation of this South American group. Centroid size does not show any reduction in the coefficient of variation over ages, while Procrustes distances within sucessive ages are reduced after 20 days. Uniform component and the more global partial warps explain most of the shape changes with age. Cranial and Facial parts of the skull increase in size at different rates with a relative lengthening of the snout and decrease in height of the braincase. We were unable to detect a clear pattern of integration for the rostrum and braincase, besides that shown by landmark displacements.