Karyotypes in large size Brazilian Oecomys vary from 2n=58 to 2n=86. A new karyotype from Rio Jamari, 2n=82, FN=106, and a new locality in Pernambuco for the 2n=60, FN=62 karyotype are reported. A summary of karyological data on large size Oecomys available in the literature is used to discuss hypothesis on speciation and karyotype evolution in this Oecomys group.
