1. Analyses made at a bath with an additional ozone processing stage showed that the bathers caused a marked bacterial contaminatin on busy days and that this contamination did not originate from the purification plant. 2. This finding and numerous individual analyses of water from various swimming pools yielded a spectrum of 14 micro-organisms, 4 genera being of frequent incidence: bacillus, staphylococcus, pseudomonas and flavobacterium. In general, bacterial contamination exists not only in the pool itself but also after filtering and even subsequent to the addition of chlorine. Staphylococci were found to be particularly invulnerable even to high chlorine contents, which is likely to be attributable to their enclosure in organic material. 5. Tenfold sampling demonstrated that the identification of Staphylococcus aureus in individual samples is unreliable despite its ubiquitous occurrence, probably due to their uneven distribution in the water, while the identification or exclusion of Pseudomonas aeruginosa proves sufficiently dependable even with only individual samples. 4. The general routine analysis of swimming pool water for the presence of staphylococcus aureus appears not justifiable, whilst testing for Pseudomonas aeruginosa appears justifiable, especially for identifying potentially dangerous filter contaminations.