In order to confirm the effect of ozonation on fouling mitigation in long-term membrane bioreactors (MBRs), parallel operation of two MBRs with and without ozonation was repeated in triplicate with different dosing modes. A number of mixed liquor properties and removal of pollutants in both MBRs were investigated. Membrane fouling was effectively retarded by ozonation in long-term MBRs. The best dosage was 0.25 mg g(-1)-SS at 1 day intervals. A slow formation of a gel layer on membrane surface was the result of the low organics in the supernatant and the low extractable extra-cellular polymeric substances in the MBR-ozone system. zonation also enlarged suspended flocs by reducing zeta-potential and increasing hydrophobicity, thus enhancing flocculability of the particles in the mixed liquor. The microbial activity was somewhat inhibited, however, the effluent quality of the system was not affected. These findings indicate the feasibility of using ozone to improve MBR efficiency. (C) 2010 Elsevier Ltd. All rights reserved.