We have identified the almost complete correspondence of whole chromosome arms between the karyotypes of Bolomys lasiurus (2n = 33, 34) and Akodon montensis (2n = 24, 25) using comparative analysis of the GTG-banding patterns. To explain the karyotypic differentiation of the species, tandem and centric fusions, pericentric inversions, loss of telomeres and centromeres are required. Localization of telomere sequences using an (TTAGGG)n oligomere as a fluorescence in situ hybridization probe provided evidence of the maintenance of interstitial telomere sequences in a polymorphic centric fusion of recent origin in Bolomys lasiurus. The homeologies shared between the chromosomes of these two species and those of A. cursor (2n = 16), from previous work, are discussed.