A tanks-in-series model was applied for mathematical modeling of the steady-state performance of continuous cultures in an airlift bioreactor. In the present computational algorithm, non-linear algebraic equations for the material balances of micro-organism, substrate and dissolved oxygen around the hypothetical well-mixed stages with backflow in the riser and downcomer were solved simultaneously using the Newton-Raphson technique. The concentration profiles of micro-organism, substrate and dissolved oxygen in the airlift bioreactor for continuous cultures were obtained under the same specifications. The simulation results were discussed mainly from the viewpoint of mixing in the airlift bioreactor. The relationships between the micro-organism productivity and design parameters of the airlift bioreactor such as liquid recycle flow rate and downcomer configuration were examined. The effect of mixing on the washout was also discussed. Numerical examples indicate that the proposed simulation scheme is very flexible and useful for the design and operation studies of airlift bioreactors.