Aerobic degradation or polishing is an essential step in the combined anaerobic/aerobic treatment of wastewater. In this study, a type of porous glass beads was used for immobilization of microbial cells in a three-phase aerobic ¯uidized bed reactor (AFBR) with an external liquid circulation. The effects of super®cial gas and liquid velocities on bed expansion, solid and gas hold-ups and speci®c oxygen mass transfer rate, k L a, were investigated. A tracer study showed that the mixing and ¯ow pattern in the 8 dm 3 reactor could be simulated by a non-ideal model of two continuous stirred tank reactors (CSTRs) in series. By treating an ef¯uent from an up¯ow anaerobic sludge blanket (UASB) digester, the distribution of suspended and immobilized biomass in the reactor as well as the kinetics of COD removal were determined. The speci®c oxygen mass transfer rate, k L a, at a super®cial gas velocity of 0.7 cm s À1 dropped by about 30% from 32 h À1 in tap water to 22 h À1 after a carrier load of 15% (v/v) was added. The measured k L a further dropped by about 20% to 18 h À1 in the wastewater, a typical value of the bubbling fermenters with no stirring. Compared with the aerobic heterotrophs under optimum growth conditions, the microbes in this reactor which was fed with anaerobic ef¯uent plus biomass behaved like oligotrophs and showed slow speci®c COD removal rates. This might be attributed to the presence of a signi®cant amount of obligate anaerobes and facultative organisms in the aerobic reactor. This was con®rmed by a relatively low intrinsic oxygen uptake rate of the microbial population in the reactor, 94 mg O 2 dm À3 h À1 or 19 mg O 2 g VS À1 h À1 .