Unambiguous species identification is fundamental, especially when attempting to study species-specific interactions or management and conservation issues. Current weasel identification methods are based on external and cranial morphology, yet North American weasels show significant geographic variation in size and considerable overlap occurs in measurements of Mustela erminea and M. frenata. As a result, these species can easily be confused. Our objectives were to : (1) develop a simple method based on restriction fragment length polymorphism (RFLP) analyses to classify Mustela spp. in eastern Québec, Canada; (2) validate existing external and cranial morphological criteria in the literature for weasel identification; and (3) calculate discriminant functions based on genetically identified specimens to aid in Mustela identification where genetic laboratory facilities are not readily available. Skeletal and external measurements were taken on adult male Mustela specimens (n = 28) that had been collected in the winter of 2002–2003. Results from genetic analyses confirmed a northern range expansion of long-tailed weasels into Québec and revealed that current morphological criteria alone are not adequate for classifying Mustela specimens. As many as 18% of Mustela erminea and M. frenata individuals were mis-identified due to morphological measurements, either falling outside known ranges for each species or inside measurement ranges common to both species.