An upflow anaerobic sludge blanket reactor was operated under thermophilic conditions (55 degrees C) for 160 days by feeding a wastewater containing sucrose as the major carbon source. The reactor exhibited a satisfactory performance due to the formation of well-settling granulated sludge, achieving a total organic carbon (TOC) removal of above 80% at an organic loading rate of 30 kg total organic C m(-3) day(-1). Structural and microbial properties of the methanogenic granular sludge were examined using scanning electron microscope X-ray analyses and serum vial activity tests. All the thermophilic granules developed showed a double-layered structure, comprised of a black core portion and a yellowish exterior portion. The interior core portion contained abundant crystalline precipitates of calcium carbonate. Calcium-bound phosphorus was also present more prominently in the core portion than in the exterior portion. Methanogenic activities of the thermophilic granules both from acetate and from H-2 increased with increasing vial-test temperature in the range of 55-65 degrees C [from 1.43 to 2.36 kg CH4 chemical oxygen demand (GOD) kg volatile suspended solids (VSS)(-1) day(-1) for acetate and from 0.85 to 1.11 kg CH4 COD kg VSS-1 day(-1) for H-2]. On the other hand, propionate-utilizing methanogenic activity was independent of vial-test temperature, and was much lower (0.1-0.12 kg CH4 COD kg VSS-1 day(-1)) than that from either acetate or H-2. Acetate consumption during vial tests was considerably inhibited by the presence of H-2 in the headspace, indicating that a syntrophic association between acetate oxidizers and H-2-utilizing methane-producing bacteria was responsible for some portion of the overall acetate elimination by the theromophilically grown sludge.