Experimental observations made on two pilot plants, showed that nitrogen removal activated sludge systems, operating under favourable conditions, seem to develop increased nitrification potential compared to fully aerobic systems under similar conditions. This increased potential, which cannot be detected by simple nitrification performance evaluations, is attributed to higher autotrophic populations sustained--developed in similar systems employing anoxic reactors or phases. A reduced autotrophic decay rate under anoxic conditions as reported by some researchers, seems to play a significant role in such a response, most likely together with a more efficient use of available nitrogen for additional nitrifying microorganisms production, resulting from a reduced nitrogen loss to autotrophic biomass maintenance needs and heterotrophic biomass synthesis requirements.