This paper describes simultaneous nitrification-denitrification (SND) in a fluidized bed reactor (FBR) without physical separation of the aerobic and anoxic zones. Continuous reactor SND trials demonstrated successful SND over distinct periods. Control was achieved by varying the reactor dissolved oxygen and the feed COD, although optimal performance could not be consistently maintained. Batch studies, using cultures from different vertical sections, demonstrated that variation of the bulk parameters successfully stratified the reactor bed into zones that predominantly either nitrified or denitrified. Evidence also indicated that both types of activity occurred throughout the FBR.