High rate granular methanogenic fermentations were performed in one-phase upflow anaerobic sludge blanket (UASB) reactors treating synthetic wastewaters containing starch, sucrose ethanol, and butyrate plus propionate. All granules formed showed high settling velocities which enabled high cell mass retention and accommodation of high loading rates. The maximum COD removal rates (g COD/l-reactor . d) obtained after 500-d operations were 7.6 for starch, 10.5 for sucrose, 32.1 for ethanol, and 42.6 for butyrate-propionate. Long-term growth on various defined substrates altered the population of bacterial trophic groups and overall characteristics of granules. The starch- and sucrose-grown granules were characterized by larger size and more abundant extracellular polymeric substances (EPS) than the ethanol- or fatty acids-grown grannies. The fatty acids-grown granules contained a considerable amount of inorganic salts (ash content : 56 to 63%) but a small amount of EPS, and showed a denser ultrastructure than the other three types of granules. The granules grown on ethanol under slightly acidic conditions showed the lowest specific gravity and volatile suspended solids (VSS) density as well as ash content among all of the granules. As aceticlastic methanogens, Methanothrix spp. were predominant in the starch-, sucrose-, and fatty acids-grown granules, whereas comparable numbers of Methanosarcina spp. were observed only in the ethanol-grown granules. The populations of hydrogenotrophic methanogens were the largest of all bacterial trophic groups in the respective granules. The data confirm that the prevalence of Methanothrix spp, and high methanogenic activity for H-2 are general characteristics of methanogenic granules and that EPS and inorganic deposits contribute chemically to the enhancement of structural stability and mechanical strength of granules.