A new genus of mylagaluid rodent from the Miocene of the Great Basin, Hesperogaulus, is described, and to it two new species, H. gazini and H. wilsoni (type species) are referred. This genus differs from all other genera of later Tertiary mylagaulines in having: large bosses of rugose bone on the anterior end of the nasal bones; lower slope of the occipital bone; and unique separation of the anterior branches of the forked parafossette on the upper premolar. Hepserogaulus n. gen. represents a distinct lineage of mylagaulids that has been separate from other members of the family since the early Barstovian. This genus is geographically limited to the Great Basin. Other later Tertiary genera of mylagaulids show geographic limitations to other parts of the continent as well.