In this work the biodegradability of displacement bleaching effluents of kraft pulping was studied. A sequential treatment with an anaerobic fluidized bed and an aerobic trickling filter was effective in degrading the chlorinated high- and low-molecular-weight material. The treatment significantly reduced the COD, BOD and the organic chlorine (AOX) of the wastewater. COD and BOD reduction was greatest in the aerobic process whereas dechlorination was significant in the anaerobic process. Less than 3% of the COD removed was generated to sludge. With the combined aerobic and anaerobic treatment over 65% reduction of AOX and over 75% reduction of chlorinated phenolic compounds was observed. The COD/AOX-ratio of the wastewater was similar before and after treatment, indicating that the chlorinated material was as biodegradable as the nonchlorinated. Microbes capable of mineralizing pentachlorophenol constituted approximately 3% of the total heterotrophic microbial population in the aerobic trickling filter. Two aerobic polychlorophenol degrading Rhodococcus strains were able to degrade polychlorinated phenols, guaiacols and syringols in the bleaching effluent.