We investigated the occurrence of abbreviated pinnae (squared ears) in little brown bats (Myotis lucifugus) and northern bats (Myotis septentrionalis) in Michigan, USA. Although the trait was not observed in 308 M. septentrionalis, squared ears occurred in 56 (0.1%) of 5,863 M. lucifugus that were examined. Squared ears were equally common in males and females. Both ears typically were affected to the same degree and most often reduced by 50% of their height, although the amount of pinna that was missing varied from 5 to 50%. Previous authors speculated that the trait was an ontogenetic malformation, but microscopic examination of squared ears from M. lucifugus in Michigan indicated that the trait consistently was associated with trauma, most likely frostbite.