Crenothrix polyspora Cohn 1870 and Clonothrix fusca Roze 1896 are two filamentous, sheathed microorganisms exhibiting complex morphological differentiation, whose phylogeny and physiology have been obscure for a long time due to the inability to cultivate them. Very recently, DNA sequencing data from uncultured C. polyspora-enriched material have suggested that Crenothrix is a methane-oxidizing gamma-proteobacterium (39). In contrast, the possible ecological function of C. fusca, originally considered a developmental stage of C. polyspora, is unknown. In this study, temporal succession of two filamentous, sheathed microorganisms resembling Cohn's Crenothrix and Roze's Clonothrix was observed by analyzing the microbial community of an artesian well by optical microscopy. Combined culture-based and culture-independent approaches enabled us to assign C. fusca to a novel subgroup of methane-oxidizing gamma-proteobacteria distinct from that of C. polyspora. This assignment was supported by (i) methane uptake and assimilation experiments, (ii) ultrastructural data showing the presence in C. fusca cytoplasm of an elaborate membrane system resembling that of methanotrophic gamma-proteobacteria, and (iii) sequencing data demonstrating the presence in its genome of a methanol dehydrogenase alpha subunit-encoding gene (mxaF) and a conventional particulate methane mono-oxygenase alpha subunit-encoding gene (pmoA) that is different from the unusual pmoA (u-pmoA) of C. polyspora.