The structure of the extracellular polysaccharide produced by the mesophilic species, Alteromonas infernus, found in deep-sea hydrothermal vents and grown under laboratory conditions, has been investigated using partial depolymerization, methylation analysis, mass spectrometry and NMR spectroscopy. The repeating units of this polysaccharide is a nonasaccharide with the following structure: [carbohydrate: see text].