Studies on solar inactivation of Escherichia coli in freshwater and in situ have been limited. At 63rd St. Beach, Chicago, Ill., factors influencing the daily periodicity of culturable E. coli, particularly insolation, were examined. Water samples for E. coli analysis were collected twice daily between April and September 2000 three times a week along five transects in two depths of water. Hydrometeorological conditions were continuously logged: UV radiation, total insolation, wind speed and direction, wave height, and relative lake level. On 10 days, transects were sampled hourly from 0700 to 1500 h. The effect of sunlight on E. coli inactivation was evaluated with dark and transparent in situ mesocosms and ambient lake water. For the study, the number of E. coli samples collected (n) was 2,676. During sunny days, E. coli counts decreased exponentially with day length and exposure to insolation, but on cloudy days, E. coli inactivation was diminished; the E. coli decay rate was strongly influenced by initial concentration. In situ experiments confirmed that insolation primarily inactivated E. coli; UV radiation only marginally affected E. coli concentration. The relationship between insolation and E. coli density is complicated by relative lake level, wave height, and turbidity, all of which are often products of wind vector. Continuous importation and nighttime replenishment of E. coli were evident. These findings (i) suggest that solar inactivation is an important mechanism for natural reduction of indicator bacteria in large freshwater bodies and (ii) have implications for management strategies of nontidal waters and the use of E. coli as an indicator organism.The survival of fecal indicator bacteria (Escherichia coli, enterococci) in ambient environments is strongly influenced by abiotic (e.g., salinity, sunlight, and temperature) and biotic (predation and competition) factors. The biological tolerance of E. coli to physicochemical factors has been especially well studied, albeit mostly in the laboratory. Of these factors, incoming solar radiation (insolation) is arguably the most potent in the inactivation or killing of E. coli and enterococci in water (6,10,17,20,21; R. S. Fujioka and E. B. Siwak, Am. Water Works Assoc., Water Qual. Technol. Conf., 1987).Studies on solar inactivation of fecal indicator bacteria have been largely restricted to marine waters (8,10,20). Little research has focused on its effect on indicator bacteria in large freshwater bodies, such as the Great Lakes. Fresh and marine waters certainly share E. coli response characteristics to sunlight, but important differences justify separate study of each system. E. coli survives longer in freshwater (9), and the response curve to its natural decline-due to predation, competition by other microflora, and deposition in sediments (3, 12-15; M. W. Rhodes and H. I. Kator, Annu. Meet. Am. Soc. Microbiol. 1984, abstr. N 41, p. 185, St. Louis, Mo., 1984-is compounded by salinity in marine water (5,11,22). Marine beaches are also subject to tidal...