A model describing the growth of a methanotrophic biofilm is presented. This model involves simultaneous growth of methanotrophs, heterotrophs and nitrifiers. Heterotrophic biomass grows on soluble polymers which arise from the hydrolysis of dead biomass entrapped in the biofilm. Nitrifiers develop because of the presence of ammonia in the mineral medium. A comparison of this model with experimental data showed that the biofilm growth, methane removal, oxygen consumption, product formation and biofilm detachment could be fitted well. Parameter estimation yielded a maximum growth rate for methanotrophs, mu m, of 1.17 +/- 0.2 d(-1), at 20 degrees C, a decay rate, b(m), of 0.34 +/- 0.06 d(-1), a half-saturation constant, K-S(CH4), Of 0.08 +/-: 0.05 mg CH4/l, and a yield coefficient, Y-CH4, of 0.21 +/- 0.03 g x COD/g CH4 GOD. In addition, a sensitivity analysis has been performed for this model. It indicated that the most influential factors were those related to the biofilm (i.e. density; solid volume fraction; thickness). This suggests that in order to improve the model, further research is needed in the field of biofilm structure and composition. (C) 1997 IAWQ. Published by Elsevier Science Ltd.