In order to determine the most probable primitive karyotype for the Peromyscus maniculatus complex and for the genus Peromyscus, G- and C-band data for P. floridanus and P. gossypinus are analyzed and compared to data available for other species. It is concluded that the primitive karyotype for the genus is characterized by chromosomes with heterochromatin restricted to centromeric regions. Heterochro-matic short arms appear to have been added independently to several Peromyscus lineages, suggesting that heterochromatin provides some selective advantage in this genus. When autosomes are identified according to the standardized karyotype for Peromyscus (Committee, 1977), the primitive karyotype is biarmed for pairs 1, 22, and 23; pairs 2, 3, 6, and 9 may or may not be biarmed, and all other pairs are acrocentric. Based on the trend in the P. maniculatus group for orthoselection, which increases the number of biarmed elements, it is hypothesized that chromosomes 2, 3, 6, and possibly 9 are acrocentric in the primitive karyotype and that P. crinitus, with only chromosomes 1, 22, and 23 biarmed, has retained an essentially primitive karyotype for the genus. The euchromatic long arm of the X chromosome has remained stable throughout all species examined, although heterochromatic short arms on the X are quite variable. Y chromosomes are biarmed and C-band positive in all species examined, but position of the centromere varies. Chromosome banding data support the hypothesis of Bowers et al. (1973) that the primitive karyotype for the P. maniculatus group is like that of P. melanotis with 30 acrocentric elements. C-bands reveal that heterochromatin in the primitive karyotype of the P. maniculatus group was confined to the centromeric regions. The primitive karyotype for this group has pairs 1, 2, 3, 6, 9, 20, 22, and 23 biarmed and all other pairs acrocentric. Peromyscus floridanus (subgenus Podomys) shares what are believed to be derived chromosomal inversions with members of the subgenus Peromyscus, suggesting evolutionary affinities between these subgenera. Chromosomal banding data appear to have considerable promise as a tool for determining clades within Peromyscus.