This study examined the relative rotation of the lower incisors of eight species of insectivorous bats from North America to test the null hypothesis that no taxonomic differences existed among the species. The lower incisors of Eptesicus fuscus, several species of Myotis, and Nycticeius humeralis (Chiroptera: Vespertilionidae) are distinctive in general shape, the degree of rotation of each tooth, and the extent of overlap of each tooth. The incisor group is wider in Eptesicus than in the other species studied. Eptesicus shows very little rotation of the incisors, but the incisors of species of Myotis show rotation up to 150 degrees from the midline of the mandible. Incisor 1 shows the least rotation and incisor 3 shows the most rotation among the three incisors in Myotis. Overlap of the incisors appears to be indirectly related to rotation, with Eptesicus showing the most overlap and Myotis the least.