Incomplete nitrification was studied in a completely and partially submerged rotating biological contactor (RBC). In a partially submerged RBC without additional aeration, 50 to 90% nitrite accumulation (alpha) was achieved at rotation speeds (omega) of 2 to 18 min(-1). In a completely submerged RBC operating during 80 days, a higher alpha of 96% was achieved at omega = 2 min(-1). Incomplete nitrification in a completely submerged RBC at oxygen concentrations of 1.5 to 6.8 mg O2/L indicated that the mass transfer of oxygen is rate-limiting. Modeling of the completely submerged RBC predicts that the oxygen profile will not penetrate the biofilm more than 30 microm, thereby strongly limiting the nitrite-oxidizer growth and causing high nitrite accumulation. Molecular analysis (i.e., fluorescence in situ hybridization) indicated that the nitrite-oxidizers are superficially located (<200 microm) and that the ammonia-oxidizers comprise up to approximately 800 microm of the biofilm.