Recent field and revisionary studies of Madagascar's endemic rodents (Muroidea: Nesomyidae: Nesomyinae) have dramatically improved our understanding of species-level diversity within this group. However, such studies have generally focused on taxa from the island's relatively well-studied eastern humid forests and have relied solely on morphometric comparisons. Herein, we undertake a study of morphometric and genetic variation within Macrotarsomys bastardi, a rodent endemic to Madagascar's dry forest habitats. In particular, we evaluate existing subspecific boundaries using comparisons of cranial and external measurements from 84 museum specimens. We then assess phylogeographic structure across the geographic range of the species using sequence variation in the mitochondrial cytochrome-b gene from a subset of these specimens. We conclude that there is little basis for recognizing established subspecies, but that molecular data reveal novel patterns of diversity and geographic structure within this species. These results, coupled with emerging patterns of diversity in other endemic Malagasy mammals, suggest that there is underestimated diversity and biogeographic structure within Madagascar's western habitats.