Aims: The objective of the present work was to describe an aerobic, mesophilic and heterotrophic marine bacterium, designated HYD657, able to produce an exopolysaccharide (EPS). It was isolated from a East Pacific Rise deep-sea hydrothermal vent polychaete annelid. Methods and Results: This micro-organism, on the basis of the phenotypical features and genotypic investigations, can be clearly assigned to the Alteromonas macleodii species and the name A. macleodii subsp. fijiensis biovar deepsane is proposed. Optimal growth occurs between 30 and 35degreesC, at pH between 6.5 and 7.5 and at ionic strengths between 20 and 40 g l(-1) NaCl. The G + C content of DNA was 46.5%. This bacterium excreted, under laboratory conditions, an EPS consisting of glucose, galactose, rhamnose, fucose and mannose as neutral sugars along with glucuronic and galacturonic acids and a diacidic hexose identified as a 3-0-(1 carboxyethyl)-d-glucuronic acid. Its average molecular mass was 1.6 x 10(6) Da. Conclusions: The bacterium HYD657, for which the name A. macleodii subsp. fijiensis biovar deepsane is proposed, produces an unusual EPS in specific medium. Significance and Impact of the Study: Due to its interesting biological activities, applications have been found in cosmetics. Its probable contribution to the filamentous microbial mat in the Alvinella pompejana microenvironment can be also mentioned.