Biofilters treating high-concentration gaseous volatile organic compounds (VOC) can be subject to bed clogging induced by excess biomass accumulation. In this study, O-3 was continuously injected into biofilters to control biomass. Its effects on the performance of the biofilters and on biomass accumulation were investigated. Four identical biofilters designed to treat gaseous toluene were operated for 70 days, and three of them were continuously injected with O-3 at different concentrations (from 80 to 320 mg/m(3)). The results showed that continuous O-3 injection could effectively keep the bed pressure drop stable and had no adverse effect on toluene removal when O-3 concentrations were 180-220 mg/m(3). The maximum toluene elimination capacity of the four biofilters was 140 g-toluene/m(3)/h, and the bed pressure drop of the biofilter fed with 180-220 mg/m(3) O-3 remained below 3 mmH(2)O/m throughout the operation period. The biomass accumulation rates of the three biofilters with O-3 at 80-320 mg/m(3) were lowered by 0.15-0.25 g/L/day compared with the biofilter without O-3. The decreases in biomass accumulation resulted in higher void fractions of the filter beds with O-3 injection. Carbon balance analysis indicated that CO2 production had increased while biomass accumulation and leachate waste production decreased in response to O-3 injection. Based on the experimental results, it was concluded here that continuous O-3 injection can reduce increases in bed pressure effectively, preserve VOC removal capacity, and prevent production of extra leachate waste.