In deposits of the late Pleistocene of Argentina the remains of ground sloths (Xenarthra, Folivora) are frequently represented by crania in which only the posterior region is preserved. In this work we characterize the occipital region of late Pleistocene Mylodontidae to allow a taxonomic identification of fragmentary specimens of Lestodon armatus, Mylodon darwinii, Glossotherium robustum and Scelidotherium leptocephalum. The available juvenile specimens were also included in this study and the ontogenetic shape changes are described. Additionally, the signs in the occipital region that allow the identification of juvenile specimens are discussed. The mylodontids G. robustum and M. darwinii have a significant variability in the occipital region which could be explained by sexual differences, or well as a result of palaeoenvironmental changes that took place during the Lujanian.