Cytogenetic studies of three subspecies of the deer mouse, Peromyscus maniculatus rubidus, P.m. gracilis and P.m. bairdii have demonstrated an intra- and intersubspecific chromosomal polymorphism in which the karyotypic patterns vary among individuals of the same and different subspecies while maintaining a diploid number of 48 and a constant karyotype in each animal. Present information suggests that pericentric inversion seems the best explanation for this polymorphism.