The chipmunks that heretofore have been assigned to the species _Eutamias quadrivittatus_ are here assigned to two species, _E. quadrivittatus_ and _E. umbrinus,_ for the following reasons: 1. The baculum of _E. quadrivittatus_ differs from that of _E. umbrinus_ in having a narrow base (see figs. 3, 4). This difference permits any specimen which has an associated baculum to be readily identified to species. 2. The cranial breadth in the subspecies of _E. quadrivittatus_ is significantly larger than in the subspecies of _E. umbrinus_. 3. Specimens of _E. umbrinus_ are darker than any specimen of _E. quadrivittatus_. 4. Where the geographic ranges of _E. quadrivittatus_ and _E. umbrinus_ come close to one another (probably they meet at some places), _E. umbrinus_ occupies a higher position in terms of life-zones. Wherever either of these two species, but not the other, occurs on a mountain the species occupies both the higher and lower life-zones. [Illustration: FIGS. 3 and 4. Baculum of _Eutamias quadrivittatus quadrivittatus_, No. 35648/47919 BS; from Canon City, Fremont Co., Colorado. Figure 3, lateral view of right side; figure 4, dorsal view. FIG. 5. Baculum of _Eutamias umbrinus umbrinus_, No. 38062; from Paradise Park, 21 mi. W and 15 mi. N Vernal, 10,050 ft., Uintah Co., Utah. Lateral view of right side. FIG. 6. Baculum of _Eutamias umbrinus montanus_, No. 20105; from 1/2 mi. E and 3 mi. S Ward, 9,400 ft., Boulder Co., Colorado. Dorsal view.] The differences between _E. quadrivittatus_ and _E. umbrinus_ are as great as, or greater than, between many species of chipmunks, such as between _E. minimus_ and _E. amoenus_, and between _E. quadrivittatus_ and _E. cinereicollis_. Although I know of no ecological differences between _E. umbrinus_ and _E. ruficaudus_, the morphological differences, as for example, differences in the structure of the baculum, and differences in color pattern, lead me to maintain _E. ruficaudus_ and _E. umbrinus_ as separate species. The present distribution of these two species is attributable to the uplift of the Rocky Mountains in the Pleistocene. That the uplift of the Rocky Mountains and the erosion which produced the present-day relief took place in Pleistocene times is supported by the evidence found by several geologists such as Hunt and Sokoloff (1950:109-123).