Quinoline degradation by Comamonas acidovorans was investigated in a three phase fluidized bed reactor at dilution rates below and above the critical value (mu(max) = 0.42 h(-1)). Quinoline was used as the sole source of carbon, nitrogen, and energy. Two attachment carriers, polyurethane foam (Bayvitec(R)) and modified cellulose (Aquacel(R)), and a gel entrapment carrier (polyvinyl alcohol) were studied and compared with regard to their effectiveness to immobilize cells. Attachment and biofilm formation was best at higher dilution rates, regardless of carrier type used. Except for the maximum biomass concentration on the carrier, Y-V (biomass per volume of solid particles), there was no significant difference in reactor performance between the investigated carriers under stationary conditions. The highest value for Y-V was found for the gel entrapment carrier (Y-V = 35 g L-1). In a long-term run (66 days), the gel entrapment carrier established a permanent biofilm on the surface of the gel beads after 900 h of cultivation time. Complete quinoline mineralization was achieved at a dilution rate of 2.0 h(-1), which is 4.7 times higher than the critical dilution rate. Identical substrate overloads were applied to the gel entrapment and the cellulose carrier by a step increase of the quinoline feed concentration at a dilution rate of 0.8 h(-1) (D similar to 2 mu(max)). The cells survived the overload, but the accumulation of quinoline and quinoline degradation products and the degradation efficiency were different for the two systems during the overload, showing the influence of the carrier type on the dynamic performance and stability of the process.