Simultaneous nitrification with denitrification was investigated using a single rotating biological contactor (RBC). The authors proposed two means to achieve simultaneous nitrification with denitrification in the single reactor: (1) to control oxygen transfer rate by reducing oxygen partial pressure (P02) in the air phase and (2) to develop a combined partially (aerobic) and fully (anaerobic) submerged RBC (CPFSR) reactor. For the former experiment, the maximum denitrification efficiency of 90 % was obtained at C:N ratio=6 and P02=0.10 atm. Moreover, heterotrophs, NH4- and NO2-oxidizers, and denitrifiers were distributed throughout the biofilm, suggesting that nitrification and denitrification can occur wheresoever the local environment meets their growth conditions. For the latter experiment, effects of type of organic matter and influent carbon:nitrogen ratio (C:N ratio) on the efficiency of simultaneous nitrification with denitrification were investigated using the CPFSR reactor. Acetate, ethylene-glycol, phenol, and poly-vinyl-alcohol (PVA) were used as carbon sources for denitrification. An excellent nitrification efficiency was obtained for all experimental runs and all organic substrates could be degraded and used for denitrification, indicating a great potential for simultaneous removals of nitrogen and xenobiotic compounds by the CPFSR reactor.