The redox control bioreactor (RCB) is a new hollow fiber membrane bioreactor (HFMBR) design in which oxygen and hydrogen gases are provided simultaneously through separate arrays of juxtaposed hollow fiber (HF) membranes. This study applied the RCB for completely autotrophic conversion of ammonia to N-2 through nitrification with O-2 and denitrification using hydrogen as an electron donor (i.e., autohydrogentrophic denitrification). The hypothesis of this research was that efficient biofilm utilization of O-2 and H-2 at respective HFs would limit transport of these gases to bulk fluid, thereby enabling completely autotrophic ammonia conversion to N2 through the co-occurrence of ammonia oxidation (O-2-HF biofilms) and autohydrogenotrophic denitrification (H-2-HF biofilms). A prototype RCB was fabricated and operated for 215 days on a synthetic, organic-free feedstream containing 217 mg L-1 NH4+-N. When O-2 and H-2 were simultaneously supplied, the RCB achieved a steady NH4+-N removal flux of 5.8 g m(-2) day(-1) normalized to O-2-HF surface area with a concomitant removal flux of 4.4 g m(-2) day(-1) (NO3- +NO2-)-N based on H-2-HF surface area. The significance of H-2 supply was confirmed by an increase in effluent NO3--N when H-2 supply was discontinued and a decline in NO3--N when H-2 supply was restarted. Increases in H-2 pressure caused decreased ammonia utilization, suggesting that excess H-2 interfered with nitrification. Microprobe profiling across radial transects revealed significant gradients in dissolved O-2 on spatial scales of 1 mm or less. Physiological and molecular analysis of biofilms confirmed that structurally and functionally distinct biofilms developed on adjacent, juxtaposed fibers.