Geographic variation of red-tailed chipmunks (Eutamias ruficaudus) in the northwestern United States was studied to determine the nature of bacular variation. Bacular differences between eastern (E. r. ruficaudus) and western (E. r. simulans) forms had suggested they might be distinct species. Chipmunks from the area where the two nominal subspecies are supposedly apposed, near the border of Idaho and Montana, prove to be typical ruficaudus. However, specimens from eastern Washington exhibit bacula that differ significantly from the typical form in seven of the eight characters studied. Discriminant function analysis unambiguously separated eastern and western bacular morphs. In another analysis, using specimens from Washington and Montana to identify “unknowns” from Idaho, each specimen from Idaho could be allocated with confidence to one or the other reference sample. Variation in cranial morphology parallels bacular variation, but is less diagnostic. Our results corroborate the view that ruficaudus and simulans may be distinct species and repeat patterns of character variation uncovered in earlier studies of this species complex.