Sunlight exposure is considered to be the most important cause of "natural disinfection" in surface water environments. The UV-B portion of the solar spectrum is the most bactericidal, causing direct (photo-biological) DNA damage. In the present experimental study, the effect of solar radiation on the elimination of bacteria in water, especially in surface water, was studied. The influence of depth and UV-B transmittance of water was determined. Comparing Escherichia coli, Enterococcus faecalis, Pseudomonas aeruginosa and Staphylococcus aureus, Enterococcus faecalis proved to be the most resistant organism. Pseudomonas aeruginosa was shown to be the most sensitive indicator bacterium among the tested microorganisms. Results show a significant correlation between radiation intensity and reduction rates. Best elimination of microorganisms occurs on the water surface; with increasing water depth, there is less UV radiation to inactivate bacteria. High turbidity substantially reduces UV-B transmittance in water causing decreased elimination efficiency. The results of the present study show that sunlight, given an appropriate intensity and good water transparency is suitable to inactivate fecal indicator bacteria within a few hours in surface waters and therefore also in bathing waters.