In this study, hydrogenotrophic methanogenesis with respect to methanogenic activity and microbial structures under extreme-thermophilic conditions were examined, and compared with the conventional thermophilic condition. The hydrogenotrophic methanogens were successfully acclimated to the temperatures of 55, 65 and 70 degrees C. Although acclimation was slower at 65 and 70 degrees.C, hydrogenotrophic methanogenesis remained fairly stable. High-throughput sequencing using 16S rRNA analysis showed that the higher temperatures resulted in single archaea community dominated by hydrogenotrophic Methanothermobacter. Moreover, the syntrophic bacteria changed from Coprothermobacter and Thermodesulfovibrio at 55 degrees C to Thermodesulfovibrio at 70 degrees C. Specific hydrogenotrophic methanogenic rate at 70 degrees C was 98.6 +/- 4.2 Nml CH4/g VS/hr, which was over 4-folds higher than that 8at 55 degrees C. The lag phase under extreme-thermophilic conditions was longer than thermophilic condition, which was probably due to the archaeal structure with low diversity. Extreme-thermophilic condition resulted in a shift in methanogenesis pathway from acetoclastic methanogenesis to hydrogenotrophic methanogenesis with the enrichment of Methanothermobacter thermautotrophicus.