The karyotypes of 31 specimens of six taxa of the genus Calomys (Rodentia, Sigmodontinae) trapped in an extensive area of Brazil (between 11o-32°S and between 46°-61oW) are reported. In the Cerrado domain, C. tener showed 2n=66 and FNa=66 karyotype with 32 pairs of autosomes, 31 of them being decreasing-sized acrocentric pairs, and one medium-to-small biarmed pair; C. expulsus showed a 2n=66 and FNa=68 karyotype, with 30 pairs of acrocentric autosomes and two biarmed elements, a submetacentric pair 1 and the medium-to-small biarmed pair also seen in the karyotype of C. tener; in Ipamerí locality (Caldas Novas, Goiás) a female with 2n=64, FNa=66 and a derivative karyotype of C. expulsus type was also observed. In the Pampas region a C. laucha female with 2n=64, FNa=68 was trapped. In addition to the two biarmed pairs seen in C. expulsus, this individual also possessed a third large biarmed submetacentric element corresponding to the largest pair of the karyotype. In the Amazon region three Calomys specimens were analyzed. Two of them depicted a cytotype similar to that of C. tener (showing nevertheless 2n=64, FNa=64 instead of 2n=66, FNa=66), with an acrocentric pair 1 and the medium-to-small sized biarmed pair, but lacking one unidentified autosomal pair. At the same locality (Pimenta Bueno, Rondônia) C. callidus presented 2n=48, FNa=66.
