The diploid chromosome number of 2n = 10 found in a new species of Akodon (Cricetidae, Rodentia) from two localities of the state of Mato Grosso, Central Brazil, represents the lowest chromosome number known for rodents. One female with nine chromosomes due to sex chromosome monosomy (2n = 9,XO) was also found. The karyotype comprises two large metacentric pairs (1 and 2); one large polymorphic pair (3), which could be acrocentric, submetacentric, or heterozygous as a result of a pericentric inversion; and one minute metacentric pair (4). The sex determination is of the XX/XY type. CBG, GTG, and RBG banding patterns, Ag-NORs, and meiotic data are presented. Fluorescence in situ hybridization with a (TTAGGG)7 repeat as a probe revealed interstitial telomeric bands (ITBs) in two of the large pairs.

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