Abstract: Two novel species of obligate methane-oxidizing bacteria, isolated from landfill soil, were characterized. Both strains were unusual in that some members of the population grew in irregularly shaped, refractile cell packets that resembled sarcina-like clusters. Electron microscopy revealed that the cell packets were covered with a slime layer and the cells contained many large granular inclusion bodies. The individual cells of each strain were sometimes motile and had differing morphologies. Isolate AML-C10T was always coccoidal in shape, and the cells were covered with extracellular fibrils. Isolate AML-D4T was pleomorphic, changing from rod to coccal form, sometimes exhibiting an unusual fusiform morphology. AML-D4T lacked the extensive fibrillar matrix observed with AML-C10T. Both strains utilized only methane and methanol as carbon sources. In stationary phase, the cells of each strain swelled in size and formed cysts. Aside from morphological differences, strains could also be distinguished from each other by cellular protein patterns, as well as by temperature and pH tolerances. 16S rDNA phylogenetic analysis showed that these are type I methanotrophs (family: Methylococcaceae) most closely related to the Methylobacter/Methylomicrobium clade, although they form a monophyletic grouping supported by moderately high bootstrap values. By 16S rDNA database searches, the most similar species to both isolates were Methylobacter spp. However, partial particulate methane monooxygenase sequence analysis suggested that these bacteria might be more closely related to Methylomicrobium than Methylobacter. Furthermore, cellular fatty acid profiles of the strains more closely resemble those of Methylomicrobium, although the absence of significant levels of 16:1omega5c argues for the uniqueness of these two strains. On the basis of the results described here, it is proposed that a new genus should be created, Methylosarcina gen. nov., harbouring two species, Methylosarcina fibrata sp. nov. (type species) and Methylosarcina quisquiliarum sp. nov. The type strains are AML-C10T (= ATCC 700909T = DSM 13736T) and AML-D4T (= ATCC 700908T = DSM 13737T), respectively.