An obligately methanotrophic bacterial strain, LW14T, isolated from the sediment of Lake Washington, Seattle, USA, is described taxonomically. The isolate is an aerobic, Gram-negative, non-motile bacterium capable of growth on methane, and possesses type I intracytoplasmic membranes (i.e. it is a type I methanotroph). The strain possesses particulate methane monooxygenase (MMO) and has no soluble MMO. Formaldehyde is assimilated via the ribulose monophosphate cycle. The isolate grows within a pH range of 4-8, with the optimum between pH 5.5 and 6.5. The cellular fatty acid profile is dominated by C(16 : )omega18c, C(16 : 1)omega7c and C(16 : 1)omega5t fatty acids. The DNA G+C content is 53.3+/-0.4 mol%. On the basis of sequence analysis of the 16S rRNA gene, isolate LW14T is related most closely to representatives of the genus Methylosarcina. However, DNA-DNA hybridization analysis reveals only a distant relationship between isolate LW14T and the previously described Methylosarcina species. On the basis of its phenotypic and genotypic characteristics, LW14T represents a novel species of the genus Methylosarcina, for which the name Methylosarcina lacus sp. nov. is proposed, with LW14T (=ATCC BAA-1047T=JCM 13284T) as the type strain.