Fecal extracts from 12 subjects in outbreaks of oyster-associated nonbacterial gastroenteritis were inoculated with BS-C-1 cells for isolation of the causative viruses. Cytopathic agents were isolated from 3 patients. No cross-neutralizing reactions were observed between the isolates and prototypes of human enteroviruses. The isolates were approximately 30 nm in diameter and had a distinct ultrastructure resembling that of astroviruses. Four polypeptide bands with molecular sizes of 42, 28, 27, and 22 kDa were seen on SDS-PAGE analyses. Seroconversion against the isolate was observed in 18 (31.6%) of 57 patients involved in five of seven outbreaks examined by neutralization test. A protein band characteristically reactive with the paired serum samples was detectable at 42 kDa by immunoblot assay. These results suggested that some small round viruses resembling astroviruses might show cytopathic effect in BS-C-1 cells and may be associated with an oyster-related gastroenteritis.