The genus Cratogeomys, particularly members of the Cratogeomys gymnurus species group, account for much of the high species diversity of pocket gophers in the Trans-Mexican Volcanic Belt. Recent molecular studies of this species group have shown strong discordance between genetically defined clades and current species taxonomy. Accordingly, we investigated relationships among the 5 species in the C. gymnurus species group using mitochondrial and nuclear DNA, chromosomes, and morphological characters. Although quantitative morphometrics provided little discrimination among species or clades within this group, the molecular data sets were consistent in identifying 5 allopatric clades, none of which corresponded to any of the 5 currently recognized species. Four of these 5 genetically defined clades lack clear diagnosability, and so are grouped into the single polytypic species, C. fumosus. The fifth clade is diagnosable based on multiple characters, including nuclear genotype, chromosomal diploid number, parasite fauna, and qualitative morphological characters. Accordingly, we resurrect Merriam's (1895) species planiceps to represent members of this clade, which occurs in the Volcán de Toluca and Valle de Bravo regions of central Mexico. Based on the observation that differences in diploid number usually signal reproductive isolation between populations of pocket gophers, we hypothesize that C. fumosus and C. planiceps are reproductively incompatible. We provide synonymies and descriptions for these 2 species, along with a key to this species group, which is now called the C. fumosus species group.