Biofilm was formed on an oxygen enrichment-type support which consisted of a poly (dimethylsiloxane) hollow fiber membrane and a fibrous support woven around the fiber. Population distribution of nitrifiers and denitrifiers in the biofilm was investigated. Nitrifiers were dominant within the fibrous support while denitrifiers were dominant in the surface biofilm attached to the support. Simultaneous organic carbon removal and nitrification were carried out successfully in single-step treatment of domestic wastewater. The nitrification rate in the present biofilm was about 2.2 g/m(2)d at an air pressure of 19.6 kPa or 29.4 kPa and was comparable to that in conventional biofilm processes designed especially for nitrification.