Population dynamics of nitrifying bacteria in an autotrophic biofilm were studied using an oxygen microelectrode. Nitrification and endogenous respiration were measured as function of oxygen concentration. Over a five weeks substrate - limiting period the shape of oxygen profiles in the biofilm had changed. The activity in the biofilm showed that the autotrophic fraction was displaced to the bulk/biofilm interface. After 5 weeks no more activity could be measured below 800 μm in the substrate-limited biofilm. Full ammonia load couldn't be buffered by those biofilms. After setting back the biofilm to full load, the profiles reached the original shape after 5 days. The results of microelectrode measurements were used to evaluate data for modelling population dynamics of nitrifying bacteria.