As a result of seriously polluted raw water sources, some water-treatment plants in China are intending to apply the biofilm process as a pretreatment prior to the traditional drinking water-treatment systems. These polluted raw waters contain higher total organic matter and ammonium than unpolluted waters. It has been reported that synthetic organic matter can cause nitrite accumulation. However, there have been few reports about the influence of fulvic acids on nitrification. Fulvic acids are the main part of the natural organic matter in waters, and nitrite is a toxic substance which has been paid attention to recently, especially in the treatment of drinking water, so it is necessary to study the effect of fulvic acids on the nitrification process. In this experiment, a so-called ＇attapulgas clay＇ biofilm reactor was set up, in which attapulgas clay was packed as the support medium for biological growth. After 40 days start-up, a multi-species biofilm developed in the reactor. The influence of fulvic acids on nitrite accumulation was investigated. Nitrite accumulation in the biofilm process was related to fulvic acids loadings. When the fulvic acids loading was less than 0.002 kg (TOC)/h.m(3), no nitrite build-up appeared; but when the loading was in the range of 0.002-0.02 kg (TOC)/h.m(3), nitrite built up and the concentration of nitrite could reach as high as 11.4 mg/l. When the loading was above 0.07 kg (TOC)/h.m(3), the nitrification process was completely inhibited.