The performance of a laboratory upflow anaerobic sludge blanket reactor (UASB-reactor) fed synthetic wastewater was investigated under organic loading rates (OLRs) ranging from 3.4 to 44.9 g COD/l . day. The distribution of substrates, intermediate byproducts, pH, VSS, specific sludge activities (acidogenic, acetoclastic, lithotrophic) and the number of different groups of microorganisms at different heights in the reactor were monitored. The conditions for active granular biomass formation were found for the OLRs investigated. When using high-glow UASB-reactors, attention should be paid to the efficient retention of the most active biomass circulating in the sludge blanket zone. The numbers of methanogens were one to two orders of magnitude higher in the lower part of the reactor (sludge bed zone) than in the upper part (sludge blanket zone). Electron micrographs of the granules showed that the predominant microbial biomass was Methanothrix. However, increasing the OLR led to a substantial increase of Methanosarcina in the granules. Significant amounts of other bacteria were found distributed within the granules between Methanothrix filaments. (C) 1996 Elsevier Science Ltd.