Hollow-fibre modules containing microporous membrane material were evaluated as bioreactors for waste gas treatment. The reactors were inoculated with the propene-utilizing strain Xanthobacter Py2, which formed a biofilm on the inner side of the fibres, The removal of the poorly soluble volatile propene from synthetic waste gas was monitored for up to 170 days. The maximum removal rates were 70-110 g propene per m(3) reactor per hour, A gas residence time of 80 s was required to remove 95% of an initial propene concentration of 0.84 g/m(3). The presence of ammonium in the liquid medium resulted in the development of an additional population of nitrifying organisms, Therefore, nitrate was used as the source of nitrogen in later experiments, During long-term operation. the propene removal rates gradually decreased, At low liquid velocities (1-5 cm/s) clogging of individual fibres with excess biomass was observed, Elevation of the liquid velocity in the fibres to 90 cm/s resulted in the formation of a dense biofilm and prevented clogging of the fibres. However, also at this high liquid velocity a gradual decrease in propene removal rate was observed, These results suggest that aging of biofilms is a very important factor in long-term operation of hollow-fibre bioreactors.