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Comparative cytogenetics studies based on conventional staining, CBG, GTG, RBG-banding, Ag-NOR staining, fluorescence in situ hybridization (FISH) using telomere probes, length measurements, and meiotic data were performed on two related but previously undescribed cricetid species referred to as Oligoryzomys sp. 1 and Oligoryzomys sp. 2, respectively, from Pico das Almas (Bahia: Brazil) and Serra do Cipó (Minas Gerais: Brazil). Oligoryzomys sp. 1 had 2n = 46 and Oligoryzomys sp. 2 had 2n = 44,44/45.Our banding data and measurements as well as FISH results support the hypothesis that the difference between the diploid numbers occurred by centric fusion events. The karyotypes had conspicuous and distinguishable macro- and micro-chromosomes, and we suppose that the largest pairs (1, 2, and 3) have evolved from a higher diploid number because of successive tandem fusion mechanisms.