A close morphological similarity among species within the Oryzomys subflavus group makes their systematic extremely difficult. Three different karyotypes have been previously detected within the O. subflavus species group in Brazil: 2n=48-50, FN=56 (Paraiba and Pernambuco); 2n=54-56, FN=62-63 (Sao Paulo and Minas Gerais); and 2n=50, FN=64 (Eastern Bahia). These three variants 1, 2 and 3 were morphologically very similar to each other. In Central Brazil (Goias and Western Bahia) we found a new

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... subflavus karyotype (variant 4, 2n=58, FN=72) which was clearly different from the other variants. The X chromosome was a large submetacentric unlike the acrocentric X chromosome in the three other karyotypes. All previously described variants contained two large biarmed chromosome pairs (the largest of their autosomal complement) which did not show counterparts in this new variant. Variant 4 was easily distinguishable by its morphological characters from any other variant. Thus, distinctive karyotypic and morphological characteristics and allopatric distribution of O. subflavus variant 4 suggest that it deserves a species status.