Quinoline degradation by Comamonas acidovorans was studied in a continuously operated three-phase airlift reactor. Porous glass beads were applied as support matrix for cell immobilization by colonization. Under steady-state conditions (S similar to 0), cell attachment was poor at low dilution rates but improved considerably with increasing dilution rate; Conversion of quinoline was investigated below and above the washout for suspended culture (D-crit = mu(max) = 0.42 h(-1)). With immobilized cells the reactor could be operated at D > mu(max), and complete conversion of quinoline was achieved as long as the specific quinoline feed rate D*S-0/X did not exceed the maximum specific degradation rate (r(S,max). The biofilm thickness was about 100 mu m, and its efficiency was about 54% compared to suspended organisms. If quinoline overloads were supplied to the reactor, quinoline, as well as its pathway intermediates, appeared in the reactor and conversion was low. Hence, the immobilized microorganisms remained viable and active. They could survive quinoline overloads. If the quinoline feed rate was reduced again, complete conversion was reestablished.