Aerobic biodegradation of a xenobiotic recalcitrant compound sodium anthraquinone-2-sulphonate (SAS), was investigated using as an inoculum a mixed microbial culture, which was activated sludge from industrial and domestic waste-water treatment plants. The difference in SAS degradation was examined using two main systems : (1) suspended cells and (2) immobilized cells, both in batch and in continuous culture. In the suspended cell system, under continuous culture conditions using SAS as a unique source of carbon and energy, it was possible to degrade about 95% of this substrate after 6 days. Maximal SAS removal rates in the suspended-cell system were 59.3 mg SAS 1(-1) h(-1) and 88.7 mg SAS 1(-1) h(-1) for dilution rates (D) of 0.05 h(-1) and 0.075 h(-1), respectively. In the immobilized-cell system, almost all SAS was degraded in 6 days and the maximal removal rate reached 88.7 mg SAS 1(-1) h(-1) at D=0.05 h(-1). Application of a continuous-flow enrichment procedure resulted in selection of several kinds of micro-organisms and led to a progressive elimination of some species of Aeromonas. A stable microbial community of 11 strains has been established and characterized at D =0.075 h(-1). Most of them were Gram-negative and belonged to the genus Pseudomonas.