An outbreak of gastrointestinal illness in which headache, low grade fever and myalgia were common symptoms occurred among persons who visited a recreational park in Macomb County, Michigan, on July 13-16, 1979. The temporal clustering of onsets of 121 persons who were the first in their households to become ill suggested an incubation period ranging from 4-77 hours. A history of swimming in the park's lake was elicited with significantly greater frequency from these persons than from park visitors who were not ill (age standardized odds ratio = 4.8; 95% confidence interval, 1.8-12.7). One hundred twenty-six park visitors who became ill were household contacts of index patients who had swum in the lake; at least 62 of these 126 cases were probably due to secondary transmission. A secondary attack rate of 19% was observed in household contacts who had not visited the park. Serologic studies identified Norwalk virus as the etiologic agent. The source of the contamination of the lake could not be determined. Although some water samples collected just before and after the epidemic period had high coliform counts, the geometric mean coliform density of all samples collected on those days was within the limits established by the Environmental Protection Agency as acceptable for recreational contact water.