Methanogenic populations were quantitatively investigated in two separate anaerobic reactors, which received effluents from acidogenesis of either food waste-recycling wastewater or swine wastewater. Each set of the anaerobic digesters was operated under high organic loading rate (OLR) conditions (8.3-39.8 g chemical oxygen demand (COD)/L/d) with four different hydraulic retention times (10.4-3.0 days). Real-time PCR analysis demonstrated that the order Methanosarcinales was the most abundant (> 55%) methanogenic group in the six runs with > 50% COD removal, while the dominance (> 70%) of Methanobacteriales was observed in the other two runs (< 20% COO removal). Methanomicrobiales constituted a minor (< 30%) population in all eight runs. The two reactor; with relatively low performance were the only trials which contained less than 5 x 10(7) copies/mL of 16S rRNA gene of Methanosarcinales. These results imply that a minimum level of Methanosarcinales is important for achieving even moderate levels of process efficiency under high OLR. (C) 2011 Elsevier Ltd. All rights reserved.