We used banding techniques to analyze the karyotypes of 17 individuals ofCtenomys dorbignyi Contreras and Contreras, 1984, from populations in Corrientes (Mbarigui, the type locality, and Sarandicito) belonging to the previously known range of the species, and Entre Rios (Tiro Federal and Paso Vera) representing a new geographically isolated group of populations. All known populations have 2n=70 although the extreme geographic isolation of the different localities suggested the possibility of chromosomal divergence. We found three different karyomorphs; two of them (Mbarigui and Sarandicito) had 6 pairs of biarmed autosomes and 28 pairs of telocentric autosomes, with an FN=80. Pair 20 (telocentric) was the carrier of the only detected NOR. However, pair 2 was clearly different in each of these two, being metacentric in Sarandicito and submetacentric in Mbarigui. The third fixed karyomorph differed significantly from the others, showing 8 pairs of biarmed autosomes and 26 pairs of telocentric autosomes, with an FN=84. Pair 3 (biarmed) was the carrier of the single NOR detected. The NOR carrier also showed a marked heteromorphism (Corrientes vs Entre Rios) evidenced by differences in the length of the short arm. These observations revealed a chromosomal polytypism that was expected owing to the disjunct geographic distribution of the species.