By comparing photographs of humpback whales (Megaptera novaeangliae) photographed at the peak of their arch during a dive, we found some to be smooth and others to be scalloped middorsally caudal to the dorsal fin. Out of 410 photographs taken in southeast Alaska and Hawaii, 66% were found to be smooth and 34% scalloped. All females with calves were smooth. One pregnant female was smooth and only one female who was pregnant was slightly scalloped. One known male was scalloped and one "singer" on the Hawaiian breeding grounds was scalloped. We suggest that the smoothness or scalloping of this region is a morphological feature and can be used with reasonable accuracy to determine sex in humpback whales.Key words: humpback whales, morphology, sex