An anaerobic, sporeforming, galactomannan-degrading bacterium was isolated from methanogenic granular sludge of an UASB reactor used for treatment of wastewater from a sugar refinery. The isolate degraded the polymer rapidly (doubling time was 55 min) and completely in mineral media. The bacterium was classified as Clostridium butyricum; the main products were acetate, butyrate, hydrogen, formate, and presumably CO2. The strain produced extracellular endo-mannanase, cell-associated exo-mannanase and intracellular alpha-galactosidase activity. The mannanases were present when grown on galactomannan, but not after growth on glucose, galactose, or mannose.