Observations were made from an elevated shore station and from small boats of membership and of changes in membership in pods of humpback whales, Megaptera novaeangliae, assembling in Hawaiian waters during the winter-spring season of 1980. Pod sizes were small, with many singletons, pairs, and triplets, and a few larger size pods. For pods having a calf present, the modal size was three: the mother, her calf, and an adult “escort” animal. When the calf was absent the modal size was two animals. On the average, 14% of the pods per hour changed membership, either losing or gaining members; for both pods with and without a calf, the modal sizes showed the lowest rate of change. The rate of change was seasonally dependent and was at its maximum between mid-February and mid-March, when the relative abundance of whales was also at its maximum. Pods with calf were much more likely to attract new members than to lose members; pods without calf were equally likely to attract or lose members. Twelve cases of photographic resightings of individuals documented changes in affiliations with intervals as short as 1 h. The driving force for the transient nature of pod membership was hypothesized to be the prospecting behavior of mature males searching for sexually mature, receptive females. The overall pattern and duration of social affiliations was consistent with characteristics of polygynous or promiscuous mating systems.