An outbreak of 33 cases of gastroenteric illnesses of unproven etiology and four cases of infectious hepatitis occurred among 128 persons who attended a picnic in August, 1966. In general, the gastroenteric illnesses developed 24 to 48 hours after the picnic, lasted one to three days, and were characterized by diarrhea, nausea, vomiting, and abdominal cramps. The illnesses were related by food histories to ingestion of raw clams (Mercenaria mercenoria) at the picnic. The four picnickers who developed hepatitis 22, 26, 37, and 40 days after the picnic had all eaten raw clams, and three had experienced the gastroenteric illness. Contamination of the clams probably occurred in the waters from which the clams were harvested. This outbreak of shellfish-associated gastroenteritis and hepatitis provides additional evidence that clams serve as a potential vehicle for the transmission of those diseases. © 1969 by The Johns Hopkins University.