The 2 subspecies of mink (Neovison vison letifera and N. v. mink) in Illinois were characterized early in the 20th century; however, quantifiable morphological distinctions between the 2 subspecies remain unclear. Illinois biogeography, and the influence of glaciations on animal populations, might explain the distinctions between the 2 mink subspecies. We evaluated sexual dimorphism within each subspecies and morphologic variation between subspecific populations. Oral, facial, and braincase regions were defined by 22 dimensions. Sexual dimorphism was present in both subspecies of mink. Data were adjusted for sex differences and tested for differences between subspecies using analysis of variance and multivariate analysis of variance. Cranial dimensions differed significantly between N. v. letifera and N. v. mink. Cranial dimensions distinguish individuals of the 2 geographically distinct subspecies; however, an intergradation zone might be present. The results are consistent with population response to glaciations and subsequent expansion, however, further genetic testing is necessary to test this historical biogeography of N. vison.