Several of the twenty or more species of pocket mice of the subfamily Perognathinae are questionable as to their systematic status and relationship to other species. In this study, three such species are examined. Perognathus amplus and P. longimembris are so similar in morphology and karyotype that at least one worker has proposed that they may be members of a single ring-species demonstrating circular overlap. On the basis of differences in morphology, karyotypes and allozymes between the subspecies of Chaetodipus penicillatus, others have suggested that this taxon may include cryptic species. The subspecies C. p. penicillatus and C. p. pricei were examined here. By examination of mitochondrial DNA, allozymes, morphology, and karyotypes of these taxa, I am able to show that: (1) P. amplus and P. longimembris are distinct taxa (species), each of which contains several distinguishable taxonomic units (subspecies). (2) P. amplus and P. longimembris are not related paraphyletically, as the circular overlap hypothesis would predict, but are two separate monophyletic groups. (3) It is possible that these two species differentiated after becoming isolated by the Colorado River when it attained its modern course. (4) C. p. penicillatus and C. p. pricei often hybridize. The hybrids are capable of backcrossing to either of the parental taxa. Thus, these two taxa are not biological species. U of I Only ETDs are only available to UIUC Users without author permission