An unusual thick-walled, sac-like structure occurs embedded in the blubber of the midventral ridge of the tail posterior to the anus, and appears to be unique to the gray whale. Data on six postanal sacs are reported. Histological examination of one sac revealed a smooth-walled, striated structure with an apparent epithelial lining. Partial chemical analysis of the contents of one sac disclosed two major components consisting of relatively homogeneous types of protein-polysaccharide complexes, at least one being a glycoprotein. Inasmuch as the structure is rather consistent in shape, routinely present, and occurs in immature and adult gray whales of both sexes, it is thought to be a naturally occurring structure rather than a tumor or parasite-induced cyst. It is possible that the postanal sacs are scent glands and function in "track laying" during migration and /or to maintain group integrity.