Lipid A was obtained in a high yield (27%) by the hydrolysis of lipopolysaccharide from the marine gamma proteobacterium Marinomonas communis ATCC 27118T with 1% AcOH. Using chemical analysis and 1D and 2D NMR spectroscopic and fast atom bombardment mass spectrometric methods, it was shown to be β-1',6-linked D-glucosaminobiose 1-phosphate acylated with (R)-3-dodecanoyl- or (R)-3-decanoyloxydecanoic acid, (R)-3-{(R)-3-hydroxydecanoyloxy)]decanoic acid and (R)-3-hydroxydecanoic acid at the C2, C2' and C3 positions, respectively. Uncommon structural peculiarities (a low acylation and phosphorylation degree) of the M. communis lipid A in comparison with those of terrestrial bacteria may be of pharmacological interest. The potential physiological meaning of this lipid A and compounds of similar structure are discussed.