The José María Arguedas' grass mouse, Akodon josemariarguedasi, is currently known from the central Yungas of Peru, ranging from the right (south) side of the Río Huallaga in Huánuco Department (in the north) to Junín Department (in the south); its phylogenetic affinities are unknown. We collected four specimens from Quintao, Tayacaja, Huancavelica Department allocated to this species, extending the species range 144 km farther south from its previous known distribution; and inferred the phylogenetic relationships of this species within the aerosus species group based on molecular analyses of the mitochondrial cytochrome b gene. We confirm that A. josemariarguedasi is a member of the aerosus species group and it is a sister taxon to Akodon mollis "Ancash" and then to Akodon torques. Also, we found that Akodon kotosh is closely related to Akodon orophilus, and not A. josemariarguedasi. Finally, we confirm that Akodon budini and Akodon siberiae are also members of the aerosus species group.