An upflow anaerobic filter (23 dm(3) working volume) was used for the treatment of palm oil mill effluent. The filter was continuously operated for 215 days with organic loads ranging from 1.2 to 11.4 kg of chemical oxygen demand per cubic metre per day and hydraulic retention times from 15 to 6 days. The overall substrate removal efficiency was very high, up to 90% and the filter effluent contained almost no suspended solids. For all runs, the operation of the filter exhibited good stability for acidity and alkalinity, indicating that the use of buffer solutions would not be required. The methane concentration in the biogas, whose production varied from 20 to 165 dm(3) per day, was about 60%. Daily gas production varied in the range 0.69-0.79 dm(3) per gram of chemical oxygen demand removed.