The narwhal, an Arctic odontocete, has two horizontally embedded teeth at birth1. In males and a few females, the left tooth erupts at the end of the first year (K. A. Hay, personal communication) and develops into a spiralled tusk, which can be up to 260 cm long. It has been suggested that the tusk is used to disturb potential benthic prey2; to pierce prey before killing3; to pierce thin ice to make breathing holes4,5; as a defensive weapon4; as a cooling mechanism6, and as a sound transmitter7,8; but it is most probably used in display or fighting9–13. There is no mention in the literature of overt aggressive behaviour between narwhals or of scars on narwhals. We have found many scars on adult male narwhals. These, together with the high incidence of broken tusks in adult males and the burst of tusk growth at sexual maturity indicate that tusks are used in aggressive encounters.