A granular-activated-carbon-packed biofilter receiving a constant loading rate of styrene was subjected to changes in residence time and concentration, and the effects on performance characteristics and the composition of biofilm along the bed height in the biofilter were studied. This study was carried out during the last 3 months of the entire biofilter operation of 16 months. The total bed height of the biofilter was physically divided into four individual reactor stages in series. This configuration permitted measurement of the leachate pH in each stage. Also, between-stage mixing of the culture was minimized. Each reactor stage was loaded in an upflow mode. The shortest residence time tested, 1.05 min, resulted in a decrease of removal efficiency to 95% (from 100% achieved at longer residence times). The shorter residence time nonetheless resulted in a higher elimination capacity in the higher stages of the filter bed. In the first two stages, the leachate pH values were 6.4 and 6.6, slightly lower than in higher stages (pH 7). A decrease of the styrene concentration along the bed height significantly affected the total cell number of immobilized cells whereas the number of degraders, Pseudomonads, and eukaryotes changed only a little. Microbial analysis of the mixed culture showed the presence of four bacterial strains and three fungi.