Intraspecific variation in chromosome morphology is greater in two species of pocket gophers than within any other presently studied mammalian species. Diploid number varies only from 76 to 78. Therefore, the majority of the differences appear to be the results of pericentric inversions. These geographical chromosomal differences in Thomomys bottae and T. umbrinus are best explained as adaptations to habitat characteristics. Within both species, populations occurring in arid or shallow soil habitats have karyotypes characterized by a high number of acrocentrics, whereas populations in more favorable conditions have karyotypes composed entirely of metacentrics or submetacentric elements. These parallel trends suggest that the symmetry of the karyotype as a whole may have functional significance.