The microstructure of granules from a full-scale UASB reactor treating brewery wastewater was examined using light microscopy as well as the scanning and transmission electron microscopies. The granules typically have a complex, layered structure. The outer layer of the granule was densely packed with various types of bacteria, including cocci, bacilli, Methanosarcina, and Methanothrix. The second layer was composed of a matrix of Methanothrix with two types of microcolony showing evidence of syntrophic associations between hydrogen-producing acetogens and hydrogen-consuming methanogens. The interior of the granule, on the other hand, was predominantly Methanothrix with scattered microcolonies of syntrophic associations, which became sparse and disappeared toward the centre core of the granule.