Effective nitrification of ammonia at high nitrogen loading rates, up to 7.5 kg N-NH4+ m(-3) d(-1) was obtained in a nitrifying activated sludge unit working at a hydraulic retention time of 1.3 h and 20 degrees C. Bicarbonate, used as a carbon source for autotrophic microorganisms, allowed maintainance of a system very stable against different shocks due to point deficiencies of oxygen or other accidents. Ammonia conversion to nitrate was normally between 97 and 99.9%. Temporary accumulations of nitrite were quickly transformed into nitrate. The relative low specific conversion rate of the biomass in the reactor (0.5-0.7 g-N-NH4+ g-VSS-1 d(-1)) was compensated by the capacity of the unit to retain high concentrations of biomass (up to 15 g-VSS l(-1)) due to the excellent characteristics of the biomass (Sludge Volume Index of 12 ml g-VSS-1 Zone Settling Velocity of 9 m h(-1)). Present results show that activated sludge units can be operated efficiently as a high-rate nitrifying technology. This well-known, robust and easy to operate technology is a clear alternative to the airlift or fluidised-bed systems, which can be applied for the nitrification of high load wastewater at full scale.