We provide new data on the karyologic patterns of the Andean rodents Aepeomys and Thomasomys, including the 1st karyotypic description of A. lugens and T. vestitus. Two differentiated chromosomal formulae were found for Aepeomys in Venezuela; they are a diploid chromosome number (2n) = 28, fundamental number (FN) = 48 karyotype corresponding to A. lugens and a 2n = 44; FN = 46 karyotype for an unnamed population (Aepeomys sp.). According to the structure of these 2 karyotypes and the distribution of constitutive heterochromatin (scarce and restricted to a few chromosomes in A. lugens versus abundant and scattered among chromosomes in Aepeomys sp.), we suggest that Aepeomys sp. is a more primitive form than A. lugens. T. laniger and T. vestitus showed karyotypes of 2n = 42, FN = 40 and 2n = 44, FN = 42, respectively; the latter has an additional pair of small-sized metacentric chromosomes and an acrocentric X chromosome (metacentric in T. laniger). Our results suggest that Aepeomys and Thomasomys are closely related taxa in terms of their karyologic patterns.