Hatchery mortality, clinical, histological and ultrastructural studies were conducted to substantially increase knowledge of larval oyster velar virus disease (OVVD). Over an 8-year period loss of up to 50% of oyster larval production in a commercial facility between March and June of each year was presumptively attributed to OVVD. The disease affects Pacific oyster (Crassostrea gigas)larvae greater than 150 mu m in shell height. Enumerative histopathological study of 900 animals demonstrated that viral inclusion bodies occur in velar epithelium in 38% of the animals studied, and in oral and esophageal epithelium in 13% and 11%, respectively. Associated lesions included cellular swelling and detachment. Intracellular lesions included swelling of the nucleus and mitochondria in association with viroplasm production.