Six subspecies of _Thomomys bottae_ occur in Colorado. _T. b. aureus_ and _T. b. howelli_ occupy the Colorado Plateau Province (see fig. 1) and are characterized by a yellowish color; nasals posteriorly truncate or rounded; posterior extensions of premaxillae long; basioccipital wide; and interpterygoid space U-shaped with a median spicule. _T. b. internatus_, _T. b. cultellus_, and a new subspecies from the vicinity of Cañon City described on page 376, inhabit the Sangre De Cristo and Wet mountains in the Southern Rocky Mountain Province and adjacent parts of the Colorado Piedmont and Raton Section of the Great Plains Province (see fig. 1). This group of closely related subspecies is characterized by reddish color; posterior margins of nasals forming a V; posterior extensions of premaxillae short; basioccipital narrow; and interpterygoid space V-shaped, lacking a median spicule. _T. b. pervagus_ occupies part of the San Luis Valley to the west of the Rio Grande (see fig. 1). In Colorado _T. b. pervagus_ is isolated from _T. b. internatus_ and _T. b. cultellus_ by the Sangre De Cristo and Culebra ranges and is separated from _T. b. aureus_ by the San Juan Mountains. _T. b. pervagus_ occupies an area geographically intermediate between _T. b. aureus_ to the west and _T. b. internatus_ and _T. b. cultellus_ to the east and has some characters in common with these subspecies. _T. b. pervagus_ resembles _T. b. aureus_ in having long posterior extensions of the premaxillae and in sometimes having rounded posterior margins of the nasals. _T. b. pervagus_ resembles _T. b. internatus_ and _T. b. cultellus_ in color, the presence of a V-shaped interpterygoid space, and a narrow basioccipital. Kelson (1951:69) has pointed out that in New Mexico the separation of the ranges of _T. b. pervagus_ and _T. b. cultellus_ is probably complete, but probably incomplete between _T. b. pervagus_ and _T. b. aureus_. Nevertheless, the similarities between _T. b. pervagus_ and _T. b. cultellus_ and _T. b. internatus_ suggest that _T. b. pervagus_ was originally derived from the more eastern stock. _T. b. aureus_ is a variable subspecies which, according to Durrant (1952:211), intergrades with _T. b. howelli_ in Utah. Specimens of _T. b. aureus_ showing the greatest amount of geographic variation cranially are from the ecotone between the Piñon-juniper and Douglas Fir zones at the edge of the range of the subspecies. _T. b. howelli_ is a markedly distinct subspecies that shows certain similarities to _T. b. aureus_, but the degree of cranial difference from _T. b. aureus_ suggests an isolation of long duration, or a rapid evolution from the parent stock.