Traditional biofilters for waste gas treatment are mainly based on the degradation activity of bacteria. The application of fungi in biofilters has several advantages: fungi are more resistant to acidification and drying out, and the aerial mycelia of fungi form a larger surface area in the gas phase than bacterial biofilms, which may facilitate the uptake of hydrophobic volatile compounds. The research described here identifies important conditions for the operation of fungal-based biofilters. Biofilters with perlite packing were operated at different pHs and relative inlet gas humidities. Toluene was used as a model pollutant. It was shown that a low pH is a prerequisite for fungal growth in biofilters. Also, the fungal biofilters were more resistant to drying out and more active than the bacterial biofilters. Fungal biofilters eliminated 80-125 g toluene/m3 filterbed/h. Several measures that could limit the clogging of fungal biofilters with fungal biomass were investigated. The introduction of mites helped to control excessive fungal growth and pressure drop. The pressure drop of a perlite/fungi/mites filter of 1 m height, loaded with 200 m3 gas/m3 filter/h stabilised around 130 Pa. Biofilters based on the action of fungi are cost-effective for the treatment of waste gases containing aromatic compounds, alkenes and other hydrophobic compounds.