Changes in the acetoclastic methanogenic activity in the effluent from an upflow anaerobic filter (UFAF) were studied throughout a 36 week operating period. The UFAF formed the second phase of a two-phase laboratory-scale anaerobic treatment system. The Specific Methanogenic Activity (SMA) test was used to measure the acetoclastic methanogenic activity of the biomass washed out from the UFAF. Throughout the operating period, the SMA tests were carried out at different organic loading rates (OLR) up to 6.7 kg COD m(-3) d(-1) and upflow velocities (UV), 5 m d(-1) and 15 m d(-1). The results showed that the biomass washed out from the UFAF consisted of up to 55% (considered as a draft value) acetoclastic methanogens producing 550 mI CH4 g(-1) VSS d(-1). This high acetoclastic methanogenic activity measured in the effluent might have been due to the existence of Methanosaeta (formerly Methanothrix) species. Towards the end of operation, Epifluorescence Microscopic examinations in the effluent revealed that Methanococcus species were the most dominant group followed by medium rods and short rods, filaments and long rods and the least dominant Methanosarcina species. The performance of the UFAF was not adversely affected by the loss of the active biomass and changes in their composition. The UFAF system achieved over 85% COD removal efficiency at the highest OLR of 6.7 kg COD m(-3) d(-1) and UV of 15 m d(-1). Throughout most of the operation, methane yield ranged from 0.28-035 m(3) CH4 kg(-1) CODremoved.