The biological treatment of some industrial wastewaters with high strength ammonium can be affected by the presence of inhibiting compounds. In the case of industries that frost bottles, this inhibitor can be fluoride. In this study, the effect of fluoride on nitrification in an activated sludge system was quantified. Working continuously in steady state, the maximum nitrification rate (MNR) was determined for different fluoride concentrations without substrate limitation. The kinetic equations for noncompetitive, Aiba and Luong inhibition models were adjusted with experimental MNR values. The non-competitive inhibition constant (K-I) was found to be 520 mgF(-) l(-1) at 20 degreesC. The only published values for this inhibition constant are for systems with immobilised biomass. The results of this work indicate that nitrification inhibition by fluoride in this suspended biomass system is higher than that observed in immobilised biomass systems in the literature. (C) 2002 Elsevier Science Ltd. All rights reserved.