In June 1977 an outbreak of acute gastroenteritis affected 103 students and teachers at an elementary school in Ohio. The illness typically lasted 24 hours or less and was characterized by vomiting (86%) and cramping (70%), but more than half of the persons involved also reported having nausea, diarrhea, and headache. Similar illness frequently followed in household members (29%) of families with primary cases. Investigation revealed that 70% of the children and teachers who swam in a pool at an all day outing June 1 (4 classrooms) and 55% of those who swam during a similar outing June 2 (2 classrooms) had the onset of acute illness from 12--48 hours later. None of the children who attended the outings but did not swim had a similar illness. The evidence suggested that the primary outbreak was caused by contaminated water in the pool and that person-to-person spread of illness followed. Results of a microbiologic study of pool water were negative for bacterial and viral pathogens. Throat washings, stool specimens, and paired blood samples studied for evidence of pathogens were negative initially, but subsequent serologic studies suggested that infection by Norwalk virus was the cause of the outbreak. The pool chlorinator which was inadvertently unconnected at the time of the school visits was reconnected and an underground leak in the water supply pipes was corrected. No more cases were reported after the pool was drained, cleaned, and reopened.