On November 17, 1993, the state health departments of Louisiana, Maryland, and Mississippi notified CDC of several outbreaks of gastroenteritis occurring in their states since November 12. Preliminary epidemiologic investigations identified consumption of oysters as the primary risk factor for illness. On November 16, the Louisiana Department of Health and Hospitals (LDHH) had identified the Grand Pass and Cabbage Reef harvesting areas off the Louisiana coast as the source of oysters associated with outbreaks in Louisiana and Mississippi. Tagged oysters associated with outbreaks in Maryland were traced to the same oyster beds. The oysters harvested from these areas had been distributed throughout the United States. On November 18 and 19, the LDHH and CDC notified state epidemiologists of the potential for oyster-associated illness; outbreaks of oyster-associated gastroenteritis subsequently were identified in Florida and North Carolina. Collaborative investigations by state health officials, the Food and Drug Administration (FDA), and CDC were initiated to determine the magnitude and characteristics of the multistate outbreak, identify the etiologic agent, and trace the oysters. This report summarizes the preliminary findings of the ongoing investigation.