Marion County, Oregon, public health investigators discovered that frozen, imported, oysters were the cause of an outbreak of gastroenteritis. The oysters had been harvested in South Korea, frozen on the half shell for seven months before being eaten raw by people who attended a reception at a medical facility. The oysters had been shipped to five U.S. states. Norovirus (genogroups I and II) was detected in an unopened carton of the implicated lot of oysters and from two ill persons. The investigation led to a recall of the contaminated oysters.