The existence of virus in fecal specimens was investigated by electron microscopy in five outbreaks of nonbacterial acute gastroenteritis. Small round viruses (SRV) were found in all outbreaks. The negatively stained particles were about 30-35 nm in diameter and with ragged edges. Morphological characteristics resembled each other. Immune electron microscopical examination revealed large aggregates of SRV particles and antibody in convalescent sera from patients of all outbreaks. Furthermore, the second outbreak caused by oysters contaminated with SRV seemed to be associated with the Hawaii agent.