whole bed height of a biofilter was divided into four individual reactor stages in series. This configuration permits a measurement of the leachate pH of each stage individually and minimizes interstage mixing of the immobilized culture. The extent to which the residence time of pollutant in the filter bed influenced biodegradation characteristics and the composition of immobilized culture under conditions of a constant loading rate was studied using a perlite biofilter having an internal diameter of 50 mm and the bed height of each stage being 27 cm. The residence time of pollutant in the bed had no influence on the removal efficiency and the elimination capacity of the whole biofilter although some changes of these parameters in the individual stages were observed. The biofilter achieved an elimination capacity of 140 gm(-3) h(-1) at removal efficiencies greater than 90%. Degradation activity decreased the pH value of the leachate to 3.5-3.0. Microbial analyses showed that styrene was degraded by eukaryotic cells at low pH values. At pH values above 4.0 prokaryotes were also present in the mixed culture.