To assess the physiological and phylogenetic diversity of culturable halophilic bacteria in deep-sea hydrothermal-vent environments, six isolates obtained from low-temperature hydrothermal fluids, sulfide rock and hydrothermal plumes in North and South Pacific Ocean vent fields located at 1530-2580 m depth were fully characterized. Three strains were isolated on media that contained oligotrophic concentrations of organic carbon (0?002 % yeast extract).Sequencing of the 16S rRNA gene indicated that all strains belonged to the genus Halomonas in the c-subclass of the Proteobacteria. Consistent with previously described species, the novel strains were slightly to moderately halophilic and grew in media containing up to 22-27 % total salts. The isolates grew at temperatures as low as "1 to 2 6C and had temperature optima of 30 or 20-35 6C. Both the minimum and optimum temperatures for growth were similar to those of Antarctic and sea-ice Halomonas species and lower than typically observed for the genus as a whole. Phenotypic tests revealed that the isolates were physiologically versatile and tended to have more traits in common with each other than with closely related Halomonas species, presumably a reflection of their common deep-sea, hydrothermal-vent habitat of origin. The G+C content of the DNA for all strains was 56?0-57?6 mol%, and DNA-DNA hybridization experiments revealed that four strains (Eplume1 Halophilic and halotolerant micro-organisms are typically isolated from hypersaline environments such as lagoons, saline and haloalkaline lakes (the Dead Sea, Great Salt Lake, thalassohaline Antarctic lakes, African rift soda lakes), artificial salterns, cured and salty foods, briny petroleum reservoirs, hypersaline desert soils and sea ice (Ventosa et al., 1982(Ventosa et al., , 1998Vreeland et al., 1980;Quesada et al., 1984;Franzmann et al., 1987a;James et al., 1990;Valderrama et al., 1991;Adkins et al., 1993;Duckworth et al., 1996 Duckworth et al., , 2000 Bowman et al., 1997;Satomi et al., 1997; Mormile et al., 1999;Bouchotroch et al., 2001;Yoon et al., 2002;Reddy et al., 2003). They have also recently been found in the very dry environment of the walls and murals of a Details of growth rates of the novel isolates under varying temperatures and salt concentrations, transmission electron micrographs of cells and a 16S rDNA-based maximum-likelihood tree including a wider range of reference species are available as supplementary material in IJSEM Online.