Computational Fluid Dynamics (CFD) can be used to investigate local reaction rates and gas-liquid mass transfer in agitated aerobic fermenters, but it is too slow for the simulation of a whole fermentation batch at present computational capabilities. This problem is addressed in the present work by developing a dynamic model with 42 subregionsfor a 0.64 m 3 pilot xanthan fermenter. The model considers local reaction and mass transfer rates in the fermenter and the change of liquid flow fields during the fermentation. Gas-liquid mass transfer fluxes are calculated based on the simplified solution of Maxwell-Stefan diffusion by assuming the mass transfer resistance in both the gas and the liquid side of the gas-liquid interface. Gas-liquid mass transfer areas are obtained from Population Balances (PB) for bubbles. The comparison of model predictions to the laboratory stirred tank experiments with aqueous xanthan shows the need of local bubble size distributions (BSD) for the description of complex gas-liquid hydrodynamics and mass transfer. The model predicts the effect of operating conditions on the temporal and spatial variation of BSDs, mass transfer coefficients, dissolved oxygen concentrations, and xanthan reaction rates in the fermenter. It can be used to investigate fermenter operating strategies. The model is also useful for the scale-up studies. 0 2006 American Institute of Chemical Engineers.