The O-specific polysaccharide of Citrobacter gillenii PCM 1542 from serotype O-12a,12b is composed of one residue each of d-glucose, d-GlcNAc, 2-deoxy-2-[( R)-3-hydroxybutyramido]- d-glucose ( d-GlcNAcyl) and two GalNAc residues. On the basis of sugar and methylation analyses of the intact and Smith degraded polysaccharides, along with 1D and 2D 1H and 13C NMR spectroscopy, the following structure of the branched pentasaccharide repeating unit of the O-specific polysaccharide was established: This structure differs significantly from that of the O-specific polysaccharide of C. gillenii PCM 1544 from the same serotype O-12a,12b, which has been established earlier (Kübler-Kiełb, J. et al. Carbohydr. Res. 2001, 331, 331–336). Serological studies confirmed that the two O-antigens are not related and suggested that strains PCM 1542 and 1544 should be classified into different O-serogroups.