The continuous maintaining of acceptable quality of purified water in industrial purified water system that is disinfected by ozone is explained by results of laboratory studies and review of 10 years results of purified water. Laboratory study revealed that biofilm bacteria Stenotrophomonas malthophilia, that constitutes the majority of biofilm in our purified water, is more than 533 times more resistant to the ozone as planctonic bacteria of the same species. Ozone at 70 ppb killed 99.98% of planctonic microorganisms (MO) in 5 minutes, however, even 450 ppb of ozone had almost no impact on biofilm bacteria in 120 minutes. After ozonization, secondary biofilm has formed faster than primary due to the fact that dead MO represent nutrient to survived ones. In spite of inefficient impact of ozone on MO in biofilm, 10 years review has shown maintaining of microbial quality of purified water which is in compliance with Ph. Eur 7.0. Excellent microbial results could be attributed to constant recirculation of water through reservoir with 70 ppb of ozone that efficiently killed planctonic MO, and diminish total organic carbon. Results of this study confirmed that resistance of MO towards ozone was not developed in investigated period.