A novel thermophilic, obligately methylotrophic, methanogenic archaeon, strain L2FAW(T), was isolated from a thermophilic laboratory-scale upflow anaerobic sludge blanket reactor fed with methanol as the carbon and energy source. Cells of strain L2FAW(T) were non-motile, irregular cocci, 0.7-1.5 mum in diameter and usually occurred singly (sometimes forming clusters of two or four cells). The cells stained Gram-negative and lysed immediately in 0.1 % (w/v) SDS. Growth was inhibited by chloramphenicol and tetracycline, but not by penicillin, bacitracin, spectinomycin, vancomycin or kanamycin. Methanol and mono-, di- and trimethylamine were used as substrates, but H2/CO2, formate, acetate, propanol, dimethyl sulfide and methanethiol were not. The temperature range for growth was 42-58 degrees C, with an optimum at 50 degrees C. The fastest growth was observed at a salinity below 100 mM NaCl; no growth occurred above 300 mM NaCl. The optimal pH for growth was 6.5; growth was observed from pH 5 to pH 7.5. The G+C content of the genomic DNA was 37.6 mol%. Analysis of the 16S rRNA gene sequence and the partial methyl-CoM reductase gene sequence revealed that the organism was phylogenetically closely related to Methanomethylovorans hollandica DMS1T (98 % similarity for the 16S rRNA gene sequence and 91 % similarity for the methyl-CoM reductase gene sequence). The DNA-DNA relatedness between L2FAW(T) and Methanomethylovorans hollandica DMS1T was 46 %. On the basis of these results, strain L2FAW(T) (=DSM 17232T=ATCC BAA-1173T) represents the type strain of a novel species, for which the name Methanomethylovorans thermophila sp. nov. is proposed.