Data on mitotic chromosomes of the 12 species of silky pocket mice ( Perognathus , subgenus Perognathus ) are evaluated in phyletic terms. Five karyotypic groups are recognized. The fasciatus group is composed of P. fasciatus, P. apache , and P. flavescens (2N = 44, FN = 48–50). Members of the flavus group ( P. flavus and P. merriami ; 2N = 50, FN = 86) are most closely related to the longimembris group ( P. longimembris, P. amplus , and P. inornatus ; 2n = 50 and 56, FN = 82–94). The parvus group ( P. alticola, P. parvus , and P. xanthonotus ) has 2N = 54, and FN = 70–104. These four species groups are allied on both karyological and morphological bases. Overall, the parvus group is considered to be the most primitive, but is closer in chromosome structure to the flavus-longimembris phyletic line than to the fasciatus line. The fasciatus group is divergent karyologically, with few apparent similarities to the other members of the subgenus. P. formosus ( formosus group; 2N = 36, FN = 52) appears not to be closely allied with the species of either the subgenera Chaetodipus or Perognathus .