C-metaphase karyotypes from bone-marrow cells from 117 individuals representing 24 populations of the pocket gopher, Thomomys bottae (Eydoux and Gervais), were studied. The geographic range of the populations sampled was California, Arizona, New Mexico and the Mexican states of Sonora and Sinaloa. No evidence of intrapopulation variation was recorded, but extensive inter-population variation in chromosome complement was observed. All individuals, excepting one, had a diploid number of 76, so that variation rested in the relative numbers of each morphological type of chromosome in the respective complements. Existing evidence suggests interfertility between adjacent populations despite degrees of karyotypic disparity.