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A histological method was demonstrated for the microscopic analysis of granules from upflowanaerobicsludgeblanket (UASB) reactors. Granules were first fixed with formaldehyde and embedded in paraffin, before being sectioned by a microtome to a thickness of 3–5 m. The section was then mounted on a glass slide for microscopic examination using phase contrast and epifluorescent excitation. Using this simple method, the microstructure of the granules and the distribution of methanogenic bacteria could be analyzed. Over 100 granules from UASB reactors treating sucrose wastewaters were examined. Microscopic photos showed that the granules had a surface layer comprised of hydrogenconsuming methanogenic cocci and bacilli, and a loosely packed interior, mainly comprised of acetotrophic Methanothrix. This simple method provides engineers with a practical tool for studying the microstructure of the granules, the mechanisms of their formation, and kinetic modeling.