Nitrifying bacteria are known to be sensitive to low pH. However, high rate nitrification (2-25 gr/liter reactor/day as N) at low pH was found in a nitrifying fluidized bed reactor with chalk as the carrier for the biomass and the sole buffer agent. Two hypotheses were suggested to explain this phenomenon (1) The nitrification only occurs  in a favorable microenvironment attached to the chalk surface (2) The nitrification is performed by acid tolerant nitrifying bacteria. These hypotheses were examined theoretically by a mass transfer model across the nitrifying biofilm, and experimentally by measuring the pH in the biofilm with microelectrodes. No favorable microenvironment was found by modeling or in the experimentally obtained pH profile. The ability of the nitrifying population to perform  high rate nitrification  at low pH was confirmed by operating nitrifying reactor with attached and suspended biomass under acidic conditions without using chalk. FISH analysis showed that ammonia oxidizers belonging to the Nitorosomonas oligotropha lineage, dominated the ammonia oxidizing population. These bacteria are known for their low k_s for ammonia. It may be concluded that N. oligotropha take over is essential for high rate nitrification at low pH.