The O-specific polysaccharide ofCitrobacter braakiiPCM 1531 (serogroupO6) was isolated bymild acid hydrolysis of the lipopolysaccharide (LPS) and found to containD-fucose, L-rhamnose, 4-deoxy-D-arabino-hexose andO-acetyl groups in molar ratios 2 : 1 : 1 : 1. On the basis of methylation analysis and 1 Hand13 C NMR spectroscopy data, the structure of the branched tetrasaccharide repeating unit of theO-specific polysaccharidewas established.