Various aspects of viscous gas-liquid modeling are discussed. A detailed model is developed for aerated fermenters. Rigorous gas-liquid mass transfer, xanthan bioreaction kinetics, and non-Newtonian hydrodynamics are combined with computational fluid dynamics (CFD). Gas-liquid hydrodynamics is investigated in a 200-dm(3) laboratory stirred tank and xanthan fermentation is studied in a 70-m(3) agitated reactor. Sub-models needed by the CFD simulation are verified against the hydrodynamic and oxygen transfer experiments. Multicomponent gas-liquid mass transfer is modeled based on the Maxwell-Stefan diffusion. A bubble swarm drag correction is developed for viscous shear-thinning fluids with a single bubble size. The laboratory stirred tank simulations predict the cavern and gas slug formation. The "snapshot" fermenter simulations show significant inhomogeneity of gas-liquid mass transfer rate, dissolved oxygen concentration (DO), and apparent viscosity of liquid in the bioreactor. Impeller flooding and poor mixing at the bottom of the fermenter were identified. The developed model can be used for the scaleup and detailed design of aerobic bioreactors.