A new genus, Methylopila, and one new species are described for a group of seven strains of facultatively methylotrophic bacteria with the serine pathway of C1 assimilation. These bacteria are aerobic, Gram-negative, non-spore--forming, motile, colourless rods that multiply by binary fission. Their DNA base content ranges from 66 to 70 mol % G + C. Their cellular fatty acid profile consists primarily of C18:1 omega 7 cis-vaccenic and C19:0 cyclopropane acids. The major hydroxy acid is 3-OH C14:0. The main ubiquinone is Q-10. The dominant cellular phospholipids are phosphatidylethanolamine and phosphatidylcholine. The new isolates have a low level of DNA-DNA homology (5-10%) with the type strains of the serine pathway methylobacteria belonging to the genera Methylobacterium, Aminobacter, Hyphomicrobium and Methylorhabdus. Another approach, involving 16S rRNA gene sequence analysis of strain IM1T, has shown that the new isolates represent a separate branch within the alpha-2 subclass of the Proteobacteria. The type species of the new genus is Methylopila capsulata sp. nov., with the type strain IM1T (= VKM B-1606T).