Pseudomonas aeruginosa (P. aeruginosa) is a relevant pathogen to bathers and, therefore, water from natural swimming pools is analyzed for its presence. P. aeruginosa can be detected regularly in surface waters. Currently the methods described in DIN EN 12780 are recommended for the quantitative analysis of water from natural swimming pools, although this standard was developed for bottled water. The present study examines whether the recommendation is satisfactory. In 93 water samples from five natural swimming pools (Naturschwimmteiche), 3226 presumptive P. aeruginosa colonies were detected. Of the 3226 colonies, 101 were directly confirmed due to their formation of pyocyanin. From the remaining 3125 colonies, 349 representative samples were isolated and examined further. Using the criteria of DIN EN 12780 fluorescense and ammonia formation from acetamide, another 45 colonies were considered to be P. aeruginosa. 71% (32) of these 45 non-pyocyanin-forming strains could not be confirmed by the APINE system. None of the 32 strains grew at 42 degrees C, which is a characteristic feature of P. aeruginosa as mentioned in a note of DIN EN 12780. In 20 of the original samples, P. aeruginosa numbers exceeded the recommended limit value of 10/100 ml. Eleven cases (55 %) were caused by P. aeruginosa, which were not confirmed taxonomically. Due to insufficient selectivity, the method according to DIN EN 12780 is not applicable for the testing of surface waters and water from natural swimming pools. The high number of false positive results for P. aeruginosa questions the standard even further. It is suggested that for surface water samples growth of P. aeruginosa at 42 degrees C has to be checked as an additional confirmation characteristics of P. aeruginosa until a reliable standard is published.