The aim of this work is to investigate the co-influence of hydrodynamics, oxygen transfer and biological activity in an oxidation ditch via the comparison of a predictive model and experimental data. Based on the continuity and motion equations for the hydraulic part and on ASM1 model for the biological part, the model allows the prediction of substrate and dissolved oxygen (DO) concentration profiles in the reactor. Nitrification and carbon oxidation reactions have been studied, in a 1.5 m(3) pilot. This model has been validated on the pilot and extrapolated on a real size oxidation ditch. The concentration profiles are sensitive to the flow velocity and oxygen transfer capacity. The simulations point out the necessity of the oxygenation characterization, particularly the correcting factors of oxygen transfer in sludge.