A nitrification process using a fluidized bed reactor with chalk (solid calcium carbonate) as the biomass carrier and the only buffer agent was studied. The pH established in the reactor varied between 4.5 to 5.5, with lower pH obtained at higher nitrification rates. In spite of the low pH, high rate nitrification was observed with the nitrification kinetic parameters in the chalk reactor similar to those of biological reactors operating at pH > 7. Results from microsensor measurements refuted the possibility that favorable pH micro-conditions prevailed on the chalk particles and contributed to high reactor performance. In addition, identification of the major bacterial species in the low pH chalk reactors revealed well-known nitrifying bacteria. Based on these results, the performance of a fluidized bed reactor with porous sintered glass particles as the carrier for the biofilm (instead of chalk particles) was tested at similar low pH for comparison purposes. In contrast to the common knowledge of the nitrifers high sensitivity to low pH, the results from the non-chalk biofilm reactor showed that well-known nitrifying bacteria have the ability to nitrify at a high rate at low pH in a biofilm reactor using an inert (sintered glass) carrier.