Steady State laboratory flow equipment is used to disinfect Coxsackievirus, E. coli, S.lutea and the plate count flora of natural water, by means of dissolved and gaseous ozone, alone and simultaneous with ultrasound (sonozone).Distilled, surface and waste water are used as media. By ozonation, a three stage inactivation kinetics is observed. Inactivation of 3 & 4 decades is obtained, at 10–95% lower ozone concentrations, and by 57–96% lower gaseous ozone dosage by the sonozonation process compared to ozonation alone. Ultrasound increases the gas–liquid ozone transfer, and decomposes the dissolved ozone. It also disaggregates the viable microbial units.