An obligately anaerobic, methanogenic archaeon, strain HC(T), was isolated from soil of the Zoige wetland on the Tibetan plateau, China. The strain was isolated through construction of an artificial butyrate-degrading consortium in co-culture with a syntrophic bacterium, 'Syntrophomonas erecta subsp. sporosyntropha' JCM 13344. Cells of strain HC(T) were irregular coccoids, 0.8-2 mum in diameter, that occurred singly and utilized only H(2)/CO(2) for growth and methane production. Growth occurred at 18-45 degrees C (optimum around 37 degrees C). The pH for growth was 5.0-8.5 (optimal growth around pH 6.6). The G+C content of the genomic DNA was 60.2 mol%. 16S rRNA gene sequence analysis indicated that strain HC(T) was affiliated to the genus Methanoculleus, with sequence similarities of 94.8-97.2 % to existing members. However, strain HC(T) was distinguished from described Methanoculleus species by not using formate for growth or methane formation and not requiring acetate as a growth factor. On the basis of phylogenetic analysis and phenotypic characteristics, the novel species Methanoculleus hydrogenitrophicus sp. nov. is proposed, with strain HC(T) (=CGMCC 1.5146(T) =JCM 16311(T)) as the type strain.