Continuous sulfate reduction at pH 4.0 was demonstrated in a pH controlled thermophilic (55 degrees C) upflow anaerobic sludge bed reactor fed with sucrose at a COD/SO(4)(2-) ratio of 0.9 and an organic loading rate of 0.8 and 1.9 gCOD (l(reactor) d)(-1) for a period of 78 days. A nearly complete sulfate reduction efficiency was achieved throughout the reactor run, corresponding to sulfate removal rates of 0.91 and 1.92 g (l(reactor) d)(-1) at sulfate loading rates of 0.94 and 2 g (l(reactor) d)(-1), respectively, by keeping the sulfide concentration below 20 mg l(-1) due to stripping with nitrogen gas. Acidification was always complete and acetate was the only degradation intermediate left in the effluent, which did not exceed 180 mgCOD l(-1) in pseudo-stationary states. The sludge was well retained in the reactor and kept its granular form. Zn, Cu, Se, and Mo accumulated in the sludge, whereas Co, Ni, Fe, and Mn leached from the sludge, despite their continuous supply to the reactor via the influent. The bacterial diversity in the reactor sludge at the end of the reactor run was low and the culture was dominated by one acidifying species, resembling Thermoanaerobacterium sp., and one sulfate reducing species, resembling Desulfotomaculum sp.