Abstract. Lateralized behaviour has been documented in non-human species, although many observers believe that it occurs at the individual rather than the population level. Its occurrence in humpback whales in Massachusetts Bay was investigated by examining active behaviour types in which preference could be given to one direction or side. These included head breaching (direction of spin), flippering (right or left), and tail breaching (direction of movement). In addition, persistent abrasions on the right or left jaw resulting from turns to one side during bottom feeding were noted. Of 75 individuals with jaw abrasions, 60 (80%) showed abrasions on only the right jaw, while 15 (20%) had abrasions on only the left. No whales had abrasions on both jaws. Location of abrasions was consistent in all resighted individuals for up to 12 years. Two of the the active behaviour types were not strongly lateralized: directional bias was seen in only five of 21 bouts (23.8%) of breaching, and in three of 11 bouts (27.3%) of tail breaching. However, 22 of 34 bouts (64.7%) of flippering showed a bias towards one direction (generally the right). Furthermore, direction of bias in all behaviour types was individually consistent between bouts and was strongly correlated with abrasions on the corresponding jaw (P = 0.0032). The sex ratio of individuals with jaw abrasions, and of those showing directional bias in active behaviour, did not differ significantly from that of the overall
