We examined patterns of size-associated morphological variation within the Red Tree Vole (Arborimus longicaudus) for the purposes of 1) identifying differences between the Red Tree Vole and the Dusky Tree Vole (A. l. silvicola), a putative subspecies of the Red Tree Vole; and 2) examining spatial patterns of morphological variation across the species' range. Our results illustrate subtle, but significant, morphological differences across the boundary that has been proposed to delimit the ranges of the Dusky Tree Vole and Red Tree Vole. However, the morphological characters examined have virtually no diagnostic utility for distinguishing between subspecies. We also performed a series of linear regressions that revealed correlations between morphological variation and latitude. Ultimately, these findings indicated that morphological variation in Red Tree Voles is in good agreement with Bergmann's Rule and Allen's Rule, 2 well-established ecographic principles that dictate relationships between environmental temperatures and morphological attributes. Because our analyses do not demonstrate strong differences between tree vole subspecies, we encourage future analyses of variation in pelage coloration and genetic structure to unequivocally determine the Dusky Tree Vole's status.