A deep-sea, mesophilic, aerobic, and heterotrophic microorganism, able to produce an extracellular polysaccharide, was isolated from a shrimp collected near an active hydrothermal vent of the Mid-Atlantic Ridge. On the basis of phenotypic and phylogenetic analyses and DNA/DNA relatedness, this strain could be assigned to the species Alteromonas macleodii as a variant of the fijiensis subspecies. It was selected for its ability to exhibit a swarming mucoid phenotype on specific media. The bacterium secreted, under laboratory conditions, an extremely viscous exopolysaccharide consisting of glucose, galactose as neutral sugars, and glucuronic, galacturonic acids as uronic acids, along with pyruvate and acetate as main substituents.