We determined the 16s rRNA sequences of the type strains of species belonging to the genera Deleya and Halomonas for which no sequence data were available previously. We also determined the 16s rRNA sequence of ACAM 21, a representative strain of a biovar of Halomonas subglaciescola. The members of the genera Deleya, Halomonas, and Halovibrio and the misnamed organism Paracoccus halodenitrijicans formed a monophyletic group within the gamma subclass of the Proteobacteria. The 16s rRNA sequences of the members of this group contained all of the signature features previously identified as characteristic of the group. The frequency of occurrence of these signature features among other members of the gamma subclass has remained stable during the expansion of the database of rRNA sequences. The levels of 16s rRNA sequence similarity between members of the species belonging to the genera Deleya, Halomonas, and Halovibrio and the misnamed organism P. halodenitrijicans ranged from 91.5 to 100%; however, the level of sequence similarity for members of well-resolved monophyletic subgroups which might represent separate genera was 98%. At a sequence similarity level of 98% 10 subgroups were resolved, but these groups could not be differentiated on the basis of chemotaxonomic or phenotypic characteristics. In this paper we propose that members of the genera Deleya, Halomonas, and Halovibrio should be placed in a single genus, the genus Halomonas, and we emend the description of this genus. While the genus Zymobacter can be clearly distinguished from the genus Halomonas, these two taxa share important genotypic, chemotaxonomic, and phenotypic characteristics. We propose that the genus Zymobacter should be transferred to the family Halomonadaceae and emend the description of the family Halomonadaceae. The 16s rRNA sequence of Halomonas subglaciescola ACAM 21 was significantly different from the 16s rRNA sequence of the type strain of Halomonas subglaciescola (strain ACAM 12) but was nearly identical to the 16s rRNA sequence of Halomonas halodurans.Species belonging to the genera Deleya, Halomonas, and Halovibrio are heterotrophic, gram-negative, rod-shaped bacteria which have been isolated from seawater, estuarine water, hypersaline soils, and bodies of hypersaline water, including Antarctic lakes and the Dead Sea. These species are aerobic, and some strains have the capacity for facultative anaerobic growth in the presence of nitrate. Members of each genus grow over a wide range of levels of salinity. All of the species require salt for optimal growth and are therefore classified as halophiles according to the definition of Larsen (17). The level of salt required for optimal growth varies among species. For example, Deleya halophila grows optimally in media containing 7.5% marine salts, Deleya salina grows optimally in media containing 5% salts, and Halomonas meridiana grows optimally in media containing 1 to 3% NaCl (13,24,31).The family Halomonadaceae was created to accomodate the genera Deleya and Halomonas (11). These tw...

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