The intact O-specific caryophyllan polysaccharide of the lipopolysaccharide fraction from the bacterium Pseudomonas (Burkholderia) caryophylli was isolated for the first time. Its structure was clarified by means of chemical and spectroscopic analysis and consisted of a homopolymer of randomly acetylated caryophyllose units. The position of acetyl groups when present is not unique: all the hydroxyl-groups on the side chain of the sugar can be substituted with a slight preference for acetylation of the C-5-C-10 tail of this unusual monosaccharide