We examined the behaviour of humpback whales (Megaptera novaeangliae) in Hawaiian waters in relation to time of day using visual observations from shore platforms and acoustic observations using hydrophones. As humpbacks are not observed to feed in Hawaiian waters, we assumed that the data were free from the influence of diel cycles of prey availability. The majority of mothers with calves were likely to be escorted by one or more male consorts only after 0700 h. The percentage of pods with three or more adults increased across the day and the rates of male-male agonistic behaviour typical of multiple-adult pods also increased. Noon observations were characterized by high rates of behaviour such as breaching that may serve as visual and/or acoustic signals. No diel variation in the number of singers was observed, suggesting that the number of singers was independent of the late afternoon peak in male-male competition within surface-active groups. The results suggest that the whales were resting in the dawn hours, behaviour related to mating begins shortly after sunrise, and that male-male competition peaked in the afternoon.