In an attempt to determine the relationship between natural bathing water quality and its effect upon the health of the bather, a series of field studies were recently undertaken. The specific objectives of the studies were: to determine what frequency of swimming might be expected from the population groups chosen; to determine what relative increase in illness incidence might be expected from swimming in even the cleanest waters; and to determine what differences in illness incidence might be expected from swimming in waters containing various degrees of bacterial pollution. The studies were conducted on Lake Michigan at Chicago, Illinois; the Ohio River at Dayton, Kentucky, and a nearby swimming pool in the same area; and finally Long Island Sound at New Rochelle and Mamaroneck, New York.