The variation pattern of the C-heterochromatin and nucleolar organizer region (NOR) distribution, both between and within two cytotypes of M. arvalis with 46 chromosomes, are described. The positive C-bands were observed on six or seven small bi-armed autosomal pairs and in two, three or four acrocentric autosomal pairs. Only a small dark C-band was observed in the acrocentric homologue from a large heteromorphic autosomal pair in the complement of specimens belonging to the obscurus cytotype. The nucleolar organizer region was localized in the secondary constrictions in three autosomal pair in the specimens of the obscurus cytotype. These results may contribute to the knowledge of cytogenetic differentiation between the cytotypes and investigation of their systematic status.