Seven novel, cold-adapted, strictly aerobic, facultatively oligotrophic strains, isolated from Antarctic sea water, were investigated by using a polyphasic taxonomic approach. The isolates were Gram-negative, chemoheterotrophic, motile, rod-shaped cells that were psychrotolerant and moderately halophilic. Buds were produced on mother and daughter cells and on prosthecae. Prostheca formation was peritrichous and prosthecae could be branched. Phylogenetic analysis based on 16S rRNA gene sequences indicated that these strains belong to the c-Proteobacteria and are related to the genus Alteromonas, with 98?3 % sequence similarity to Alteromonas macleodii and 98?0 % to Alteromonas marina, their nearest phylogenetic neighbours. Whole-cell fatty acid profiles of the isolates were very similar and included C 16 : 0 , C 16 : 1 v7c, C 17 : 1 v8c and C 18 : 1 v8c as the major fatty acid components. These results support the affiliation of these isolates to the genus Alteromonas. DNA-DNA hybridization results and differences in phenotypic characteristics show that the strains represent a novel species with a DNA G+C content of 43-45 mol%. The name Alteromonas stellipolaris sp. nov. is proposed for this novel species; the type strain is ANT 69a T (=LMG 21861). An emended description of the genus Alteromonas is given.The genus Alteromonas belongs to the c-Proteobacteria and was created by Baumann et al. (1972) for marine, Gramnegative, heterotrophic bacteria that are motile by a single, polar flagellum. On the basis of 16S rDNA sequence analysis, the genus was revised in 1995 to contain a single species, Alteromonas macleodii, and the remaining species were reclassified as Pseudoalteromonas (Gauthier et al., 1995). In 1993, the yellow-grey-pigmented species 'Alteromonas rava', which is able to produce a novel antibiotic, was described (Kodama et al., 1993), but the species name has not yet been validly published. A mesophilic, heterotrophic bacterium, isolated from sea water that was collected near a deep-sea hydrothermal vent, was identified as A. macleodii, but it was classified as a novel subspecies, 'A. macleodii subsp. fijiensis', on the basis of a relatively low DNA-DNA hybridization level (<90 %, but >70 %), metabolic differences between the type strain and the novel strain, the ability of the novel bacterium to produce a unique exopolysaccharide and the isolation source (Raguénès et al., 1996). The subspecies name 'A. macleodii subsp. fijiensis'has not yet been validly published. Raguénès et al. (1997) proposed a novel Alteromonas species, 'Alteromonas infernus', for a polysaccharide-producing bacterium that was isolated from the surface of the vestimentiferan worm Riftia pachyptila, which inhabits sites near hydrothermal vents. The name of this novel species, however, has also not been validly published. Romanenko et al. (1994) described a novel species, Alteromonas fuliginea, but phylogenetic analysis based on 16S rDNA sequence data revealed that this species is related more closely to Pseudoalteromonas haloplank...