In this research I evaluated E. coli O157 strains (SF-sorbitolfermenting and NSF-non sorbitolfermenting) ability to survive in samples of surface waters. These samples were additionally contaminated with other strains, used as background microflora. These water samples were stored in the temperature 5 degrees C and 25 degrees C in laboratory and environmental conditions. The results of the research show changes of phenotype and antigen properties of E. coli O157 strains during research. These changes make it difficult to isolate E. coli O157 strains from natural water with numerous background microflora. These strains in surface water are threat for humans, who swim such waters in the summer The threat results from toxic virulent factors staying for a short period of time in bacterial cells, which are lost in water