To degrade toxic org. compds. (e.g., PhOH with Pseudomonas putida) in a steady-state process, use was made of a heterogeneous fermentation model of a stirred tank bioreactor, involving partial thickening and recirculation of the biomass and the interphase oxygen transfer, to study reactor yield in relation to liq.-phase av. residence time (0-30 hrs), aeration intensity (0-0.2 m/s), and biomass thickening deg. (0-0.7). Steady-state curve branch courses enabled the safe reactor operation region and the "catastrophic" sets (zero production) to be estd., important in reactor design and operation practice. Elevated thickening degrees enable the reactor to be more loaded with the toxic carbon reactant and to be run at higher feed rates.