Laboratory studies showed that Pseudomonas aeruginosa, Aeromonas hydrophila and Acinetobacter anitratum are more resistant to chlorine and ozone than Escherichia coli. Pseudomonas aeruginosa was the most resistant of all strains tested and was selected as test organism for pilot plant studies on the efficiencies of chlorine and ozone disinfection. The disinfectants proved equally efficient for the disinfection of chlorine demand-free water but ozone was much more efficient in the case of water with a chlorine demand. This difference was confirmed to be due to the property of chlorine to react with nitrogenous substances to form chloramines which are less efficient disinfectants than chlorine. Ozone does not share this property. Laboratory studies showed that ozone was highly efficient in the chemical oxidation of the pesticides parathion, fenthion, lindane and dieldrin and the detergents Manoxol OT and LAS. These compounds are not oxidized under conventional conditions of chlorination. Consideration of the above findings and other advantages and disadvantages of chlorine and ozone accentuate the value of using appropriate combinations of disinfectants for the treatment of certain waters. A final treatment sequence of ozonation, activated carbon filtration and chlorination may improve costs and efficiency of water reclamation.