The performance and microbial community structure of a pilot-scale upflow anaerobic sludge blanket (UASB) reactor inoculated with flocculent sludge were investigated over 52 days. The characteristics of corn-ethanol wastewater were as follows: CODCr, 1,050-4,970 mg l(-1); ammonia, 14-298 mg l(-1); and alkalinity, 332-2,867 mg l(-1). The UASB could start up smoothly with a hydraulic loading rate lower than 180 l h(-1) and a ratio of volatile fatty acid versus alkalinity between 0.04 and 0.48. The maximum gas production rate was 432 l h(-1) and the highest volumetric loading rate of 7.2 kg m(-3) day(-1) was obtained after 48 days. The 1 mm granules could form a complex network and were composed of many Methanosaeta. Aceticlastic methanogens served as a dominant methanogenic group, which accounted for the relatively high resistance to shock loading.