A dynamic model (pseudohomogeneous growth model) describing growth and substrate consumption of immobilized Nitrosomonas europaea resulted in predictions for consumption rates that were higher than experimentally obtained data. This overestimation was caused by effects of diffusion limitation over microcolonies. Diffusion limitation over microcolonies has been integrated in the colony expansion model. In this model growth is implemented by expansion of colonies. Colonies breaking through the gel surface cause release of bacteria into the surrounding medium. Two options were considered for this release : either growth resulting in continuous leakage of single bacteria or eruption of entire colonies at once. In the latter case both macroscopic consumption rates and transient-stare biomass concentration profiles are described satisfactorily by the model. It is also shown that the results of the previous experiments with Nitrobacter agilis can be described by the model as well.