A juvenile female California sea lion (Zalophus californianus) was found beached on the north coast of California in March, 1980. She was weak and emaciated and during hospitalization developed hemorrhagic gastroenteritis, incoordination in the hind-flippers and finally, on day 26, a watery green diarrhea. At feeding the following day the animal was eating well, stopped suddenly, breathed rapidly, and refused to eat further. She died the following morning. On post-mortem examination there was hemorrhagic fluid in the stomach, and the liver was congested and filling the right caudal abdomen. Histologically a diagnosis of hepatitis was made. Eosinophilic intranuclear inclusion bodies were found and electron microscopy revealed virions morphologically classifiable as adenovirus. No attempts at viral isolation have been made.