An anoxic biofilm involved in continuous denitrification process was monitored to investigate the effect of different concentrations of influent dissolved oxygen (DO) or nitrite on the biofilm. Microelectrode measurements evidenced nitrate removal activity of biofilm. When different concentrations of DO were applied to the reactor, generally decreased concentrations of DO were observed as bed depth increased from the bottom of the reactor. Greatest decrease of the DO was observed in the lower 20% of the bed depth. Nitrate removal efficiency was inversely proportional to influent DO concentrations (8.3-11.9 DO mg L(-1)) or nitrite loading rates (0-5.5 N-NO2- kg m(-3) day(-1)) employed in this study. Nitrite loading rates to achieve more than 90% of nitrate removal efficiency were 1.46 N-NO2- kg m(-3) day(-1) or less at pH 7.5 and 0.34 N-NO2- kg m(-3) day(-1) or less at pH 6.8. Nitrate removal efficiency was 63% or more within the lower 20% of the bed depth at the nitrite loading rates that allowed more than 90% of nitrate removal efficiency of the reactor. The results of this study provide first quantitative data that nitrate removal performance of an anoxic biofilm is inhibited by DO or nitrite, reported to be a limiting factor in the suspended biological denitrification process.