© 2018 Walter de Gruyter GmbH, Berlin/Boston. The phylogenetic position of the Ancash leaf-eared mouse Phyllotis definitus remains unresolved, as this species has been considered to be related to Phyllotis magister, Phyllotis xanthopygus or Phyllotis andium by several authors. The objective of this study was to clarify the phylogenetic position of P. definitus using 801 base pairs of cytochrome b sequences. A matrix constituted by 43 specimens was subjected to maximum likelihood, maximum parsimony and Bayesian inference analyses. Our results indicate that P. definitus is a member of the "andium/amicus" group and is closely related to Phyllotis pearsoni, supporting a set of chromosomal and morphological characters previously identified for this group.