A dynamic model for growth and substrate consumption of immobilized cells is evaluated by comparing experimental and calculated pseudo-steady-state oxygen-concentration profiles in the support material. For this, Nitrobacter agilis cells were immobilized and cultivated in agar slabs. The system was operated as a continuous submerged-bed reactor. At different growth states oxygen-concentration profiles were determined in the slabs with a microsensor. It was shown that simulated and measured profiles agreed very well, both in steady and in pseudo-steady states.