Capsular polysaccharide was isolated by the phenol–water extraction of Acinetobacter baumannii ACICU cells and studied by sugar analysis, partial acid hydrolysis, and 1D and 2D 1H and 13C NMR spectroscopy. The polysaccharide was found to contain 5,7-diacetamido-3,5,7,9-tetradeoxy-l-glycero-l-manno-non-2-ulosonic or di-N-acetylpseudaminic acid (Pse5Ac7Ac), and the following structure of the branched tetrasaccharide repeating unit was established: [Display omitted] The genes present in the polysaccharide gene cluster of A. baumannii ACICU are appropriate to the structure established.