Batch operation with different starting substrate concentrations (biological oxygen demand (BOD5)) and biomass concentrations (mixed liquor volatile, solid (MLVS)) were performed on a completely mixed three-phase fluidized bed reactor used in treating simulated domestic waste water. The mixed culture microorganisms used were contained in a biofilm attaching to cement balls. The data obtained were fitted to five different kinetic rate equations. The kinetic parameters of each model were obtained using a Gauss-Newton nonlinear regression analysis method. It was found that the Haldane model incorporating endogenous metabolic consumption (Endo-Haldanc model), best described the kinetics of the biological reaction inside the three-phase fluidized bed reactor. This is due to the fact that this model can both account for the effects of cell death and/or endogenous metabolism at low concentration and of substrate inhibition at high concentration. (C) 2002 Elsevier Science Ltd. All rights reserved.