Photographs of individually identified humpback whales Megaptera novaeangliae were collected in regions throughout the central and eastern North Pacific during the years 1977 to 1983. A comparison of these photographs revealed extensive movement between seasonal habitats. Whales found wintering near Hawaii traveled to summer feeding regions throughout the coastal waters of Alaska. Whales wintering near Mexico were found in Alaskan feeding regions and near the Farallon Islands off central California. Little exchange was found between the 2 wintering grounds or among the 5 summering grounds studied. Fidelity to a given feeding region was demonstrated by a high proportion of migratory return. Evidence of fidelity to a given wintering ground was less conclusive. The coloration of humpback whale flukes showed a longitudinal cline across the 5 feeding regions. Flukes of whales from the easternmost feeding regions were, on average, darker than those from the westernmost feeding regions. Whales in Hawaii and Mexico were similar in fluke coloration and the average coloration on both wintering grounds was intermediate between the extremes of the feeding regions. We propose that humpback whales in the eastern and central North Pacific form a single structured stock consisting of several geographically-isolated 'feeding herds' which intermingle on 1 or more wintering grounds. Mark-recapture analyses of resighting data indicate that the Hawaiian wintering congregation is 4 to 6 times larger than the southeastern Alaska feeding herd. Within a structured stock, sets of whales interact with different probabilities in each seasonal habitat. This, in turn, has important implications for the social organization and management of these whales.