Arvicanthis, the unstriped grass-rat, is a widespread genus occurring in many regions of Africa and is major pest in agricultural farmland. Despite its economic importance for developing countries, the taxonomy of the genus is stillin a chaotic state. We used univariate and multivariate morphometrics to investigate two species, A. abyssinicus and A. dembeensis, occurring in Ethiopia. Results show that these taxa are well separated, this contradicting authors who lump all Arvicanthis as A. niloticus. There is a longitudinal cline in the morphology of A. dembeensis from eastern regions along the Rift Valley. Differences between the species in morphlogy seem to reflect adaptation to different ecological niches at high (A. abyssinicus) and low (A. dembeensis) altitudes.