The majority (63%) of harbour porpoises stranded around the Moray Firth, Scotland, died from trauma characterized by multiple skeletal fractures and damaged internal organs. Surface injuries consisted of skin cuts resembling the teeth marks inflicted by one cetacean on another. The spacings between these matched those between teeth in bottlenose dolphins, of which there is a population in the Moray Firth. Four violent dolphin-porpoise interactions have been witnessed. Reasons for these interactions are unknown and similar documented examples between other mammals are extremely rare. These findings challenge the benign image of bottlenose dolphins and provide a hitherto unrecorded cause of mortality in porpoises.