A 16-yr-old adult male northern fur seal (Callorhinus ursinus) was found dead in its outdoor pool in November 1995. The animal was maintained at Mystic Aquarium (Mystic, Connecticut, USA) from March 1980 to November 1995. Gross necropsy findings included hemoperitoneum and locally extensive gastric intramural hemorrhage that involved the posterior fundic, antral, and pyloric regions and extended into the duodenum. The gastric mural thickening grossly resembled hemangioma, and the gastric serosa was ruptured at the site of maximal mural expansion. In histologic sections of the stomach, a cribiform network of fibrin, which encompassed numerous variably-sized aggregates of closely packed erythrocytes, markedly expanded the submucosa. No vascular endothelium was identified in serial histologic sections of the expanded gastric submucosa stained with hematoxylin and eosin or immunohistochemically with antibodies to vimentin and Factor VIII-related antigen, establishing an absence of hemangioma. Carstairs' and Weigert's histochemical stains confirmed that the framework expanding the submucosa was fibrin. Although the appearance of the gastric wall resembled hemangioma, a population of neoplastic endothelial cells was not identified within the submucosal expansion of hemorrhage and fibrin, and microscopic evidence was most consistent with the diagnosis of gastric intramural hematoma. This lesion is a rare pathologic event that has not been reported in marine mammals, but one that should be included in diagnostic considerations of hemoperitoneum and gastric mural expansion.