An aerobic methanotrophic bacterium was isolated from an acidic (pH 3.9) Sphagnum peat bog in north-eastern Russia and designated strain MG30(T). Cells of this strain were Gram-negative, pale pink-pigmented, non-motile, thick rods that were covered by large polysaccharide capsules and contained an intracytoplasmic membrane system typical of type I methanotrophs. They possessed a particulate methane monooxygenase enzyme (pMMO) and utilized only methane and methanol. Carbon was assimilated via the ribulose-monophosphate pathway; nitrogen was fixed via an oxygen-sensitive nitrogenase. Strain MG30(T) was able to grow at a pH range of 3.8-7.3 (optimum pH 5.8-6.4) and at temperatures between 8 and 30 °C (optimum 20-25 °C). The major cellular fatty acids were C16:1ω5t, C16:1ω8c, C16:1ω7c and C14:0; the DNA G+C content was 48.5 mol%. The isolate belongs to the family Methylococcaceae of the class Gammaproteobacteria and displayed 94.7-96.9% 16S rRNA gene sequence similarity to members of the genus Methylomonas. However, strain MG30(T) differed from all taxonomically characterized members of this genus by the absence of motility, the ability to grow in acidic conditions and low DNA G+C content. Therefore, we propose to classify this strain as representing a novel, acid-tolerant species of the genus Methylomonas, Methylomonas paludis sp. nov. Strain MG30(T) (=DSM 24973(T)=VKM B-2745(T)) is the type strain.