In order to determine the impact of sunlight on the reduction rate of indicator bacteria, bathing waters of different turbidity were artificially contaminated with indicator bacteria and exposed to sunlight. The reduction rates determined for the indicator bacteria showed a significant correlation with radiation intensity and, thus, also the time of day as well as the turbidity of the water. Under conditions of strong radiation intensity and low water turbidity (< 5 FTU), indicator bacteria were eliminated within 1-2 h of contamination with an initial concentration of 10(5) CFU/mL. Moreover, differences between different indicator bacteria were found. Enterococcus faecalis proved to be significantly more resistant to sunlight than the other indicator bacteria tested (Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus). The results of the present study, namely the complete elimination of indicator bacteria within a few hours, showed that sunlight, given an appropriate intensity and good water transparency, was the most important factor in the reduction of hygienically-relevant microorganisms in surface waters and, thus, also in small-scale bathing ponds.