Math. model of an airlift bioreactor for aerobic biodegradn. of org. substances was developed and used for anal. of 3 hydrodynamic regimens of its operation. The hydrodynamic riser, downcomer, degassing and demersal zones of the bioreactor were taken into consideration. Computer simulations were performed to det. the steady states and concn. profiles of the reagents the bioreactor along.