Key biological processes such as nitrification do not result from the work of a single bacterial species but are performed by a wide variety of bacteria. However, this microbial diversity is usually not tracked during reactor operation and mostly neglected in present mathematical models, which distinguish at the most between ammonium oxidizers and nitrite oxidizers, assuming the same properties for all bacteria of each group. Nevertheless, experimental evidence is available that different process conditions may favour the selection of different types of bacteria. This contribution deals with observed differences between two nitrifying inverse turbulent bed reactors (ITBRs) with different solid hold-ups. It also contains additional experimental data, revealing a microbial Population shift upon changing the operating conditions in one of these reactors, The reactor behaviour is described with a basic two-step nitrification model, which includes the competition between two different types of ammonium oxidizers, besides nitrite oxidizers. Considering the oxygen concentration as the key variable governing the population shift, the influence of microbial growth parameters oil the competition dynamics is assessed. (c) 2008 Elsevier Ltd. All rights reserved.