A thermophilic, methylotrophic methanogen, strain ZC-1(T), was isolated from the Shengli oilfield, China. Cells of strain ZC-1(T) were motile cocci, 0.7-1.0 microm in diameter and always occurred in clusters of two to four cells. Lysis-susceptibility experiments and analysis of transmission electron micrographs of strain ZC-1(T) suggested the presence of a proteinaceous cell wall. Strain ZC-1(T) used methanol, methylamine and trimethylamine as substrates for methanogenesis. Optimal growth, with a doubling time of around 5 h, occurred at pH 6.0-6.5, 65 degrees C, 0.3-0.5 M NaCl and 0.05-0.20 M MgCl(2). The DNA G+C content of this organism was 56 mol%. Analysis of 16S rRNA gene sequence and the inferred amino acid sequence of the mcrA gene of strain ZC-1(T) indicated that it is related specifically to members of the family Methanosaetaceae (90.6 and 76.6 % sequence similarity, respectively). However, strain ZC-1(T) failed to grow with acetate as substrate for methanogenesis, which is a special characteristic of the family Methanosaetaceae. Based on these phenotypic and phylogenic characteristics, strain ZC-1(T) is proposed to represent a novel genus and species, for which the name Methermicoccus shengliensis gen. nov., sp. nov. is proposed. The type strain is ZC-1(T) (=CGMCC 1.5056(T)=DSM 18856(T)). Methermicoccaceae fam. nov. is also proposed.