Introduction : Recently, the use of culture-independent molecular phylogenetic techniques has greatly increased our inventory of archaeal and bacterial diversity at deep-sea vents (e.g. Harmsen et al., 1997; Reysenbach et al., 2000; Takai et al., 2001) and several of the lineages appear to be endemic to deep-sea vents (Reysenbach et al., 2000b; Takai et al, 2000; Longnecker & Reysenbach, 2001). Using environmental data, some of these lineages have now been cultivated (L'Haridon et al., 1998; Campbell et al., 2001). For example, until recently, the chemolithoautotrophic and thermophilic bacterial lineage, the Aquificales, was restricted to shallow marine vents and terrestrial thermal springs. However, as part of two independent studies, one in Yellowstone National Park and a second exploring diversity associated with deep-sea hydrothermal vents, two environmental 16S rRNA gene sequences were identified that grouped within the Aquificales (pBB and VC bac27, Fig 1) (Reysenbach et al, 2000a; Reysenbach et al., 2000b). These two bacterial sequences were both related to Aquifex, yet different enough (>15% difference in 16S rRNA sequence) to comprise a different genus and perhaps even a different family. Using the ecological information gleaned through in situ biogeochemical experiments in Yellowstone (Reysenbach et al., 1999), it was concluded that members of this novel lineage are likely microaerophilic hydrogen-oxidizers. Subsequently, the first isolate representing this lineage was obtained from deep-sea vents at 9° 50'N on the East Pacific Rise (Reysenbach et al, 2000c). This isolate has been named Persephonella marina Gotz et al., 2002. In this report we demonstrate that this novel lineage in the Aquificales occurs at deep-sea vents in the Mid-Atlantic and Indian Oceans, and together with other deep-sea isolates, forms a distinct lineage from the terrestrial isolates and sequences.