Acute gastric dilatation with volvulus occurred in a 4-year-old female northern fur seal (Callorhinus ursinus). The animal had been maintained in an outdoor exhibit at Mystic Marinelife Aquarium (Mystic, Connecticut, USA) from June 1989 to March 1994. On necropsy, we observed a 360 degrees clockwise rotation of the stomach about its mesenteric axis as viewed ventrodorsally. The stomach was markedly distended with gas, fluid, and partially digested fish. The spleen followed the rotation of the stomach 360 degrees and was enlarged and congested with twisted mesentery. This is the first reported case of acute gastric dilatation with volvulus in a pinniped.