Two species of Otomys, O. unisulcatus (2n = 28) and O. irroratus (2n = 28) were karyotypically compared by G-banding, C-banding and silver staining. The genomes of the two species differ in respect to all three cytogenetic parameters examined. Although no G-band homologies were evident between O. unisulcatus and O. irroratus, the former species, on comparison with the murid, Rattus norvegicus, and the New World cricetid, Peromyscus melanophrys, shows the conservation of several primitive cricetid-murid G-band sequences. The extensive changes in the O. irroratus genome have resulted from numerous structural rearrangements to the point where neither the changed patterns nor events causing them are identifiable. It is argued that narrow habitat specificity and its susceptibility to climatic vagaries has, through population fractioning and the creation of isolated demes, facilitated rapid chromosomal evolution in O. irroratus.