Lesions were identified within the oral cavity and on the lips of 25 of 27 adult sea otters examined at one of the otter rescue centers operating during the Exxon Valdez disaster response effort. Lesions consisted of variably sized, irregular, white plaques and deep, often bilaterally symmetrical, ulcers. In severely affected animals, the lesions coalesced to cover extensive areas of the labial, buccal, gingival, and glossal mucosa. Histopathological examination of lesions revealed chronic ulcers with associated superficial bacterial colonies, and separate foci of ballooning degeneration and necrosis of the mucosal epithelium. Numerous Cowdry type A intranuclear inclusion bodies were evident in degenerative and necrotic cells and viral particles consistent with a herpesvirus were demonstrated ultrastructurally. We speculate that the viral lesions progressed to chronic ulcers caused by bacterial infection. Similar, but less severe, lesions with associated herpetic inclusions were subsequently identified in otters in the wild. Attempts to isolate the virus have been unsuccessful. This is the first report of a viral disease in sea otters.