Phylogenetic relationships among individuals of 9 taxa of the Akodon varius group of sigmodontine (tribe Akodontini) rodents from Argentina, Bolivia, and Paraguay were examined using nucleotide sequence data from the entire 1,140 base pairs of the mitochondrial cytochrome-b gene to clarify the evolutionary relationships among lineages. Two individuals of A. iniscatus were included to test proposed relationships with the A. varius group. Maximum-parsimony, minimum-evolution, maximum-likelihood (TVM I G), and Bayesian analyses revealed 2 major clades, 1 composed of Yungas Forest taxa and 1 of non-Yungas (lowlands) taxa, and 8 well-supported terminal clades. The Yungas Forest clade includes A. glaucinus, A. simulator, A. tartareus, and A. varius, which are recognized as distinct species. A. iniscatus was the most basal taxon in the lowlands clade containing A. dayi, A. toba, and A. dolores. Results support the proposed conspecificity of A. dolores and A. molinae, and the relationship within the A. varius group, which had not been clearly defined, is clarified. A. dayi and A. toba were found to be more closely related to A. dolores than to the Yungas Forest clade with which they historically have been associated. A low level of percent sequence divergence between A. toba and A. dolores suggests the need for a closer examination of this relationship, as does the high level of within-clade percent sequence divergence for A. dayi.