Oligoryzomys chacoensis

Karyotype: 2n = 58 and FN = 74. Autosomal complement: nine metacentric and submetacentric pairs (one large and remaining medium to small), and 19 acrocentric pairs (one large and the remaining medium to small). Sex chromosomes: X, a large submetacentric; Y, a medium acrocentric (Myer & Carleton 1981, pp. 16, Fig. 5B).

The karyotypes of O. brendae and O. chacoensis were morphological different, despite the fact that they share the same diploid and fundamental number.