During attempts to obtain novel, human-associated species of the domain Archaea, a coccoid micro-organism, designated strain B10(T), was isolated in pure culture from a sample of human faeces collected in Marseille, France. On the basis of its phenotypic characteristics and 16S rRNA and mcrA gene sequences, the novel strain was classified as a methanogenic archaeon. Cells of the strain were non-motile, Gram-staining-positive cocci that were approximately 850 nm in diameter and showed autofluorescence at 420 nm. Cells were lysed by 0.1% (w/v) SDS. With hydrogen as the electron donor, strain B10(T) produced methane by reducing methanol. The novel strain was unable to produce methane when hydrogen or methanol was the sole energy source. In an atmosphere containing CO(2), strain B10(T) could not produce methane from formate, acetate, trimethylamine, 2-butanol, 2-propanol, cyclopentanol, 2-pentanol, ethanol, 1-propanol or 2,3-butanediol. Strain B10(T) grew optimally with 0.5-1.0% (w/v) NaCl, at pH 7.6 and at 37 °C. It required tungstate-selenite for growth. The complete genome of the novel strain was sequenced; the size of the genome was estimated to be 2.05 Mb and the genomic DNA G+C content was 59.93 mol%. In phylogenetic analyses based on 16S rRNA gene sequences, the highest sequence similarities (98.0-98.7%) were seen between strain B10(T) and several uncultured, methanogenic Archaea that had been collected from the digestive tracts of a cockroach, a chicken and mammals. In the same analysis, the non-methanogenic 'Candidatus Aciduliprofundum boonei' DSM 19572 was identified as the cultured micro-organism that was most closely related to strain B10(T) (83.0% 16S rRNA gene sequence similarity). Each of the three treeing algorithms used in the analysis of 16S rRNA gene sequences indicated that strain B10(T) belongs to a novel order that is distinct from the Thermoplasmatales. The novel strain also appeared to be distinct from Methanosphaera stadtmanae DSM 3091(T) (72.9% 16S rRNA gene sequence similarity), another methanogenic archaeon that was isolated from human faeces and can use methanol in the presence of hydrogen. Based on the genetic and phenotypic evidence, strain B10(T) represents a novel species of a new genus for which the name Methanomassiliicoccus luminyensis gen. nov., sp. nov. is proposed. The type strain of the type species is B10(T) ( = DSM 24529(T) = CSUR P135(T)).