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 gamma-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)omega7c, C(17 : 1)omega8c and C(18 : 1)omega8c 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 69aT (=LMG 21861T=DSM 15691T). An emended description of the genus Alteromonas is given.