Baculum of _E. r. ruficaudus_: Shaft thick; keel proportionally low, 1/4 of length of tip; tip 31 to 33 per cent of length of shaft; angle formed by tip and shaft 120°; distal 2/5 of shaft slightly compressed laterally; base slightly widened; shaft long, 4.09 to 4.56 mm. Differs from _E. r. simulans_, in keel proportionally lower, tip proportionally shorter, distal 1/5 of shaft less laterally compressed, shaft usually longer; from _E. cinereicollis_, in angle formed by tip and shaft smaller, shaft shorter, ridges on either side of tip distinct; from _E. quadrimaculatus_, in tip proportionally longer, shaft usually shorter; from _E. speciosus_, _E. panamintinus_, _E. umbrinus_, _E. palmeri_, and _E. bulleri_, in base not markedly widened, shaft longer, angle formed by tip and shaft larger. For comparisons with _E. alpinus_, _E. minimus_, _E. townsendii_, _E. sonomae_, _E. amoenus_, _E. dorsalis_, _E. merriami_, and _E. quadrivittatus_, see the accounts of those species. Baculum of _E. r. simulans_: Shaft thick; keel proportionally low, 2/5 of length of tip; tip 40 to 48 per cent of length of shaft; angle formed by tip and shaft 130°; distal 2/5 of shaft laterally compressed; base slightly wider than shaft; shaft medium to long, 3.30 to 4.26 mm. Differs from _E. cinereicollis_ in, keel proportionally higher, shaft shorter, tip proportionally longer, angle formed by tip and shaft smaller, distal 2/5 of shaft more laterally compressed; from _E. quadrimaculatus_ in, keel proportionally higher, angle formed by tip and shaft larger, tip proportionally longer, distal 2/5 of shaft more laterally compressed, shaft shorter; from _E. speciosus_, _E. panamintinus_, _E. umbrinus_, _E. palmeri_, and _E. bulleri_, in base not markedly widened, shaft usually longer. For comparisons with _E. alpinus_, _E. minimus_, _E. townsendii_, _E. sonomae_, _E. amoenus_, _E. dorsalis_, _E. merriami_, and _E. quadrivittatus_, see the accounts of those species. The differences between the bacula of the subspecies _Eutamias ruficaudus ruficaudus_ and _E. r. simulans_ are comparable to those usually found between species of chipmunks. Consequently, I suspect that _E. r. simulans_ and _E. r. ruficaudus_ are specifically distinct and suggest that a search would be worth while for specimens in the geographic area between the geographic ranges as now known for the two kinds to ascertain whether intergradation (the criterion of subspecies) occurs. I suppose there is no intergradation but in the absence of precise information, I choose not to modify the current taxonomic arrangement of _E. r. ruficaudus_ and _E. r. simulans_. _Specimens examined_: 17. _E. ruficaudus ruficaudus_: Montana: _Flathead Co._: 1 mi. W and 2 mi. S Summit, 5,000 ft., 2. _Ravalli Co._: Big Hole Hill, 6,000 ft., 2; Big Hole Hill, 6,600 ft., 1; Continental Divide, Big Hole Hill, 7,000 ft., 1.