Oryzomysis the genus of South American cricetine rodents with the greatest number of species. More than 50 nominal forms have been assigned to the genus, but the taxonomical situation of most of these still confusing.Oryzomysalso shows a great chromosomic differentiation and it karyotypes have been used as a diagnostic character for its species. With the aim of determinating the taxonomical frontiers between the species of themacconnelli-capitogroup, the banding karyotypes of two morphologically similar species of this group:O. talamancaeandO. capitowere studied. The karyotype ofOryzomys talamancaeis very different from that of the other species of this genus, because it has few elements (34) and they are all biarmed. On the other hand, the karyotype ofO. capito, constituted by 54 chromosomes, mostly acrocentrics, is rather ordinary among the oryzomyine. G-banding was performed with tripsin treatment, and C-bands by denaturation-renaturation. The comparison of the G-banding patterns allowed us to detect at least three chromosomic arms shared by both karyotypes. The C-banding pattern ofO. talamancaeis very peculiar because the heterochromatin was found only on the centromeres of two autosomic pairs and on the X chromosome. These results suggest that chromosomic differentiation between the two species has occurred via Robertsonian fusions.