In this article we are reporting two cytotypes found in representatives of Zygodontomys genus trapped in two localities of Brazilian Amazon. From the locality of Surumú (Roraima) were analyzed 12 individuals (cytotype 1) which showed 2n = 86 / FN = 96–100, presenting the two largest pairs (1 and 2) as subtelo-submetacentrics, 4 or 5 medium to small biarmed pairs, the remainder being acrocentric. In Tartarugalzinho (Amapá) 7 specimens which displayed 2n = 84 / FN = 96–98 (cytotype 2) were caught. In both cytotypes the sexual pair is composed by a large submetacentric X chromosome and a median subtelocentric Y chromosome. By G-banding the largest pairs as well as the sex chromosomes could be identified. The C-bands occurred at the centromere of the majority of the autosomes and the short arm of the X and the whole Y chromosomes were heterochromatic. The NOR-bands were prevalently seen at the short arm of one medium and one small acrocentric pairs. Comparing the individuals studied in Brazil with those reported in Venezuela is observed that, although the same or similar diploid numbers, the morphology of the chromosomes of these taxa is quite different, the karyotypes of the specimens from Brazil having a smaller number of biarmed elements.The sexual pair is distinct also, the difference basically concerning to the size of the Y chromosome. Whether these karyotype differences may correspond to different morphological taxa is a subject that deserves further investigations.