Neuroanatomical sex differences have been documented in the rat neocortex, including dimorphism of its predominant commissure, the corpus callosum (CC). In particular, CC sex differences have been reported in the ultrastructure of the posterior callosal region, the splenium. Since the CC is a heterogeneous fiber tract with its axons arising from distinct cortical areas and passing through restricted regions along its length, it became of interest to ascertain whether cellular sexual dimorphisms may also be present in another division of the CC. To test this hypothesis, electron microscopy was used to examine axon composition in adult male and female rats in the anterior portion, the genu. The number and size of axons, the thickness of the myelin sheath, and the area within the genu occupied by these constituents, were quantified. Results showed a significant sex difference in the ratio of unmyelinated to myelinated axons, with females having a larger proportion of unmyelinated fibers. This effect was present for both (1) the number of axons, and (2) the area taken up by axonal fibers. No differences were found in the size of either axon type, or for myelin thickness. Comparison of these results with those from the splenium and possible mechanisms underlying this dimorphism are discussed.