Anaerobic treatment of a kraft bleaching plant effluent was studied with focus on the removal of chlorinated organic compounds and biochemical O-2 demand (BOD). Experiments were carried out using a laboratory-scale anaerobic fixed-film process operated at different hydraulic retention times (HRTs). The process efficiently removed a variety of chloro-organic compounds when the HRT was 10 h or longer. However, a very long treatment time was required for significant removal of chloroform; in fact, at the shortest HRT tested (6 h), there was a net production of chloroform. The overall removal of organically bound chlorine, measured as adsorbable organic halogens, was about 50%, and a 60% removal of the extractable organic chlorine was achieved at an HRT of 15h. About 70% of the BOD, and 20% of the chemical O-2 demand (COD) were removed. Up to 0.19 Nl of methane was produced per gram of COD removed by the treatment. When the HRT of the anaerobic process was shortened from 10 to 6 h, the methane production ceased.