Eighty-four specimens of Oryzomys subflavus, collected in the State of Pernambuco, Brazil, were studied. A Robertsonian chromosome polymorphism, characterized by a varying diploid number of 50, 49, 48, and 46, was found. All the specimens showed a chromosome arm number of 56. G-banding patterns in somatic cells allowed identification of the chromosome pairs (2, 3, 5, and 7) involved in centric fusion. C-banding revealed the presence of constitutive heterochromatin near the centromere the X chromosome and those of the autosomes. The Y chromosome presented a large heterochromatic block in the distal portion of its long arm.