Sexual dimorphism in cranial morphology was studied in a population of Oxymycterus rutilans from the Paraná Delta, Argentina. Individuals were assigned to three categories on the basis of toothwear. Condylobasal length, length of the diastema, maximum incisor width and length of the maxillary molar row provided quantifiable differences among age groups. The stepwise discriminant analysis showed sexual dimorphism in the cranial dimensions for all age classes of O. rutilans specimens. Cranial length measurements maximised the differences between sexes, with males’ being longer than females’ for all age classes.