The K98 capsular polysaccharide (CPS) from the Acinetobacter baumannii clinical isolate, REV-1184, was studied by sugar analysis and Smith degradation along with one- and two-dimensional H and C NMR spectroscopy and high-resolution electrospray ionization mass spectrometry. The CPS was found to consist of linear tetrasaccharide repeats (K-units) that include one residue each of D-GlcpNAc, D-GalpNAc, 2-acetamido-2-deoxy-D-galacturonic acid (D-GalpNAcA), and 2-acetamido-2,6-dideoxy-d-glucose (N-acetylquinovosamine, D-QuipNAc), with the GalpNAc residue decorated with a (R)-configurated 4,6-pyruvic acid acetal group. The CPS has a similar composition to that of A. baumannii K4 but the topology of the tetrasaccharide K-unit is different (linear in K98 versus branched in K4). This was due to a difference in sequence for the Wzy polymerases encoded by the CPS biosynthesis gene clusters KL98 and KL4, with the Wzy polymerase forming a β-D-QuipNAc-(1→3)-D-GalpNAc linkage between the K98 units.