The inherent operational problems of biofilters such as a pressure drop increase and nutrient limitations were managed in a toluene-removing gas biofilter with a sustainable grazing fauna consisting of micrometazoa and ciliate protozoa. Dynamic populations of predatory nematodes (Caenorhabditis sp.), rotifers (Philodina sp.), tardigrades (Echiniscus sp.) and fly larvae represented the micrometazoa community in the filter bed. Colpoda inflata, Euplotes harpa and Acineria sp. constituted the grazing ciliate community. The spatiotemporal distribution and abundance of the grazing fauna depends on physicochemical conditions and interspecies interactions in the biofilter. Of the micro metazoa, Caenorhabditis and Philodina tolerated wide concentration ranges for toluene (0.75-2.63 g m(-3)) and CO(2) (0.92-6.08 g m(-3)) and maintained stable populations of 3.4-4.7 x 10(3) and 5.8-7.65 x 10(4) g medium(-1), respectively. The grazing fauna supported a stable toluene-degrading bacterial community composed of four Pseudomonas spp. Under a maximum toluene load of 120.72 g m(-3) h(-1), at steady-state conditions 80% toluene removal was achieved in the biofilter. Of the grazing organisms, owing to their reproductive cycle and feeding behaviour, fly larvae were not suited for application in the biofilter. Meanwhile, organisms such as nematodes, rotifers and ciliates capable of tolerating a wide pollutant concentration range and maintaining a sustainable population are ideal candidates for application in biofilter technology.