A full-scale upflow anaerobic sludge blanket (UASB) reactor was investigated in terms of archaeal composition, acetoclastic methanogenic capacity and performance over a 2-year period. Performance of the reactor in terms of COD removal efficiency varied between 60% and 80% at organic loading rates (OLRs) in the range of 2.5-12 kg COD m(-3) d(-1). The reactor had been operated under a F/M (food to microorganisms) ratio of 0.02-0.03 gCOD gTVS(-1) d(-1), which is much lower than the typical values reported for similar reactors. According to specific methanogenic activity (SMA) tests the anaerobic sludge was operating at only 12-34% of its potential acetoclastic methanogenic capacity. These results demonstrated that the UASB reactor was under loaded compared to its maximum loading capacity. All other operational parameters had been maintained within their desired ranges. The SMA test and Fluorescence in situ hybridization ( FISH) results revealed that a decrease in the acetoclastic methanogenic activity of the UASB sludge from 344 mL CH4 gTVS(-1) d(-1) to 109 mL CH4 gTVS(-1) d(-1) coincided with a decrease in the relative abundance of acetoclastic Methanosaeta from 90% +/- 1.2 to 79% +/- 1.4 of the archaeal population, and an increase in the relative abundance of hydrogenotrophic Methanobacteriales from non-detectable levels to 24% +/- 0.7% of the archaeal population during the 2-year operation of the reactor. The relative abundance of archaeal cells within the UASB sludge was in the range of 15-17%.