Biomass accumulation is a major obstacle for long-term, stable operation of biotrickling filters treating high loadings of volatile organic compounds (VOCs). Clogging reduces pollutant removal and increases the pressure drop in biotrickling filters. Several options exist to remove excess biomass or to slow down the accumulation rate, but so far none has succeeded in combining a high VOC removal rate with a low biomass net growth rate. Recently, we observed the invasion of some of our biotrickling filters by small flies. In the best cases, a rapid reduction of the biomass content was observed. The fly larvae rapidly spread throughout the reactor and biomass was rapidly removed from the packing, initially at a rate of 13.1 kg wet weight/m(3). day and increasing up to 70-140 kg/m(3) of reactor day. In that case, the wet biomass content in the reactor was reduced from 455 to 28 kg/m(3) of reactor in 16 days with 80% of the biomass reduction occurring in 2-4 days. Analysis of the recycle liquid indicated that the major mechanism of biomass removal was detachment of biofilm, although experiments are underway to determine the exact proportion of biofilrn detachment and consumption by larvae. We speculate that larval activity loosened the biofilm structure, thus enhancing biofilm detachment by shear-stress from the trickling liquid. Overall, the preliminary results presented herein highlight that the use of fly or other larvae presents a tremendous potential for controlling biomass in biotrickling filters.