Peatlands are important sources of CH(4) emissions to the atmosphere and molecular surveys have identified a diverse, but mainly uncultured, euryarchaeal community in them. Characterization of a strain, E1-9c(T), associated with uncultured group E1, from a minerotrophic fen is reported. Cells were regular cocci, usually found in pairs, that stained Gram-positive and were resistant to lysis by 0.1 % SDS. Multiple flagella were observed, but motility was not observed in wet mounts. Optimal growth was obtained at moderate temperatures (28-30 degrees C) and slightly acidic pH (5.5). Total Na(+) and NaCl were only tolerated at concentrations less than 100 mM and 0.5 %, respectively, and Na(2)S concentrations above 0.1 mM were inhibitory. H(2)/CO(2) and formate were the only methanogenic substrates used by E1-9c(T); formate concentrations above 50 mM were inhibitory for growth. Vitamins, coenzyme M and acetate (4 mM) were required for growth and the doubling time was about 19 h. Phylogenetic analysis of the 16S rRNA gene and inferred McrA amino acid sequences showed that E1-9c(T) represented an independent lineage within the order Methanomicrobiales. Physiological and phylogenetic comparisons with different members of the order supported classification of E1-9c(T) in a new genus in the Methanomicrobiales. The name Methanosphaerula palustris gen. nov., sp. nov. is proposed; strain E1-9c(T) (=ATCC BAA-1565(T) =DSM 19958(T)) is the type strain of Methanosphaerula palustris.