Removal of mono-chlorobenzene (m-CB) vapors from air streams in a biotrickling filter unit was studied under various operating conditions concerning inlet m-CB concentration (C-gi), air residence time (tau), liquid recirculation rate (Q), pH, and frequency of medium replenishment. Removal of m-CB was high under all conditions especially when compared to the performance of conventional biofilters removing compounds similar to m-CB. The effect of pH was found to be much less than what is observed with suspended cultures of the same biomass. Concentration profiles of m-CB along the filter bed suggest either zero-or first-order rate law for the process, depending on the inlet m-CB concentration value.