The Mylagaulidae are a family of extinct fossorial rodents that are common in the North American Miocene. Alphagaulus pristinus, from the Barstovian of Montana, was previously known only from partial dentaries and isolated teeth, with no well-described skull or postcranial material. Alphagaulus is important to understanding the evolution of burrowing in mylagaulids, because it is within this genus that the most dramatic evolution of fossorial morphology within mylagaulids occurs. The other three species of the genus are all known from partial skulls, rare isolated postcrania for Alphagaulus vetus, and, in the case of Alphagaulus tedfordi, a nearly complete skeleton. We describe new material housed at the University of Washington Burke Museum, including three well-preserved skulls and partial skeletons showing different ontogenetic stages from juveniles to an adult. The description of this new material gives insights into the ontogeny of these burrowing mammals. In particular, it is apparent that the developmental trajectory mimics evolution in the increase of fossorial modifications of the skeleton with time. The description of these specimens also allows reanalysis of the phylogenetic relationships within the Mylagaulidae, confirming that the genus Alphagaulus is paraphyletic. Though the phylogenetic positions of species within the genus are different, our results are more stable than those of previous analyses, allowing greater confidence in the reconstructed relationships among mylagaulid species and genera.