In this work we examined the geographic genetic structure of the subterranean herbivorous rodent Ctenomys australis (sand-dune tuco-tuco) using mitochondrial DNA (mtDNA) control region (D-loop) sequences. We found 24 haplotypes in the analysis of 70 individual 403–base pair sequences; most were restricted to single populations, although a few haplotypes were shared broadly across the species' range. Genetic differentiation was not consistent with a simple model of isolation by distance, possibly evidencing a lack of equilibrium between gene flow and local genetic drift. The analysis of mismatch distributions, Fu's F_S-test of neutrality and the “starlike” topology of the gene genealogy showed a pattern consistent with a recent population expansion event. However, we could not rule out an alternative explanation based on departures from strict neutrality of mtDNA.