he aim of this study was to observe the efficiency of ozone transferred by an airstone bubble diffuser, us- ing two ozone generators with different output of ozone (5 g.h–1 ‒ G1; 15 g.h–1 ‒ G2). The retention of ozone in ozonised distilled and potable water and the devitalisa- tion effects on E. coli in the water were also noted. Ozone was introduced to two types of potable water of differ- ent composition intended for mass consumption, (MC)a and (MC)b, distilled water, and well water intended for individual consumption. The devitalisation effect of ozone on E. coli in well water (WW) and added to pota- ble and distilled water was observed. The results of our study showed that under the conditions used, the level of ozone during ozonisation with G1 increased more rap- idly in distilled water and after termination of ozonisa- tion, the retention of ozone in distilled water was a little lower in comparison with the potable water. The devital- isation of E. coli either naturally present in the water or added to it required the level of ozone close to or equal to 0.25 mg.l–1