The presence of significant denitrification activity in an aerobic toluene-treating biofilter was demonstrated under batch and flow-through conditions. N2O concentrations of 9.2 ppm(v) were produced by denitrifying bacteria in the presence of 15% acetylene, in a flow-through system with a bulk gas phase O-2 concentration of > 17%. The carbon source for denitrification was not toluene but a byproduct or metabolite of toluene catabolism. Denitrification conditions were successfully used for the reduction of 60 ppm(v) nitric oxide to 15 ppm(v) at a flow rate of 3 L min(-1) (EBRT of 3 min) in a fully aerated, 17% v/v O-2 (superficially aerobic) biofilter. Higher NO removal efficiency (97%) was obtained by increasing the toluene supply to the biofilter.