Aims: To examine the mechanism of ozone-induced damage to cytoplasmic membrane and cell ultrastructure of Pseudomonas aeruginosa ATCC27853. Methods and Results: Cell suspensions of Ps. aeruginosa ATCC27853 were treated with ozonated water. The leakages of cellular potassium (K(+)), magnesium (Mg(2+)) and adenosine triphosphate (ATP), determined by inductively coupled plasma/mass spectrometry (ICP/MS) and a commercial bioluminescence assay kit, were to assess ozone-induced damage to the cytoplasmic membrane. Maximum leakages of K(+) and Mg(2+) were attained, respectively, at 0.53 mg l(-1) ozone after 0.5 and 2 min with >99% inactivation of culturable bacteria, while that of ATP was achieved at 0.67 mg l(-1) ozone after 1 min. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed that treated cells retained intact shapes and cytoplasm agglutinations and vacuoles occurred. Conclusions: Ozone inactivates Ps. aeruginosa ATCC27853 by the combined results of increased cytoplasmic membrane permeability and cytoplasm coagulation, rather than by severe membrane disruption and cell lysis. Significance and Impact of the Study: Pseudomonas aeruginosa is a common water-related pathogen. These insights into the leakage of cytoplasmic components and ultrastructural changes provide evidence for the mechanisms of ozone-mediated inactivation.