Specimens ofOryzomysaff.subflavuscollected in Central Brazil displayed a karyotype with 2n= 58 and FN = 70, not described previously. Sex chromosomes polymorphisms were identified: the X could be a large submetacentric (Xa), a large subtelocentric (Xb) or a large subtelocentric smaller than Xb (Xc) and the Y appeared as a medium submetacentric (Ya), a medium subtelocentric (Yb) or a medium acrocentric (Yc). G- and C-banding, as well as chromosomal measurements were used to establish the rearrangements involved in the sex chromosomes variability.