A juvenile California sea lion (Zalophus californianus) died 28 days after admission to the California Marine Mammal Center. Necropsy revealed hemorrhagic fluid in the stomach and hepatomegaly. Histologically, there was evidence of multifocal necrotizing hepatitis and acute suppurative bronchopneumonia. Amphophilic intranuclear inclusions were found, and electron microscopy revealed virions morphologically classifiable as adenovirus.