The toxic and/or inhibitory effects of 4-chlorophenol (4-CP) on the total treatment efficiency of a medium strength synthetic wastewater and the effect of hydraulic retention time (HRT) on the fate of 4-CP in a laboratory-scale upflow anaerobic fixed-bed reactor were investigated. The experiments demonstrated that 4-CP inhibited the oxidation of the growth substrate, glucose. The total treatment efficiency of 5000 mgCODl(-1) glucose, decreased from 97.4 for a 4-CP-free operation to 31.8% for an initial 4-CP concentration of 200 mg1(-1) at an HRT of 2.5 days. However, deterioration in biogas quality started even at lower concentrations (at 50 mg4-CP1(-1)). In terms of 4-CP, the only concentration, at which > 99% efficiency was achieved, was 10 mg 1(-1). At a constant glucose concentration of 5000 mgCOD1(-1) and 4-CP of 100 mg 1(-1), increasing HRT to 10 days caused a reduction in treatment efficiency, most probably, due to the starvation conditions. Maximum 4-CP and chemical oxygen demand removal efficiencies were obtained at an HRT of 5 days with 58-97.4% respectively. The biogas production rate increased from 0.51 1 d(-1) at an HRT of 10 days to 5.74 1 d(-1) at an HRT of I day. (C) 2002 Elsevier Science Ltd. All rights reserved.