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Mitochondrial-DNA sequence data were analyzed from individuals sampled from 38 localities across the complete geographic range of the closely related pocket gopher genera Pappogeomys and Cratogeomys. Results of phylogenetic analysis of 1133 base pairs from the cytochrome b gene are consistent with past hypotheses of relationships among members of the castanops species group within the genus Cratogeomys. However, phylogeographic variation within the gymnurus species group of the genus Cratogeomys differs significantly from relationships reflected by current taxonomy. The data indicate that there are five geographically distinct clades within the gymnurus species group. Members of the two nominal species C. gymnurus and C. tylorhinus are scattered among these clades. The three peripherally isolated species, C. fumosus, C. neglectus, and C. zinseri, do not appear to be genetically distinct from other gymnurus species group taxa. An historical biogeographic hypothesis is proposed that will be tested using nuclear DNA data. [Copyright &y& Elsevier]