The data presented here provide strong evidence that support the hypothesis that the narwhal's tusk is used during fights between mature males. Evidence of violent fighting among adult males includes high numbers of head scars, broken tusks, and a tusk tip embedded in a skull. Females and immature males had significantly fewer head scars, and immature males had a significantly lower incidence of broken tusks. Among mature males, scar number was positively related to body girth, tusk weight, and tusk girth, but not to tusk length or body length. Relating dominance to the pattern of many head scars on massive mature males with massive tusks is consistent with data from other species for which both anatomical and behavioural observations are available. The lack of significant correlations of tusk length with body length and body girth in mature males is not consistent with the hypothesis that dominance assessment is based on a display of tusk lengths.