Acinetobacter baumannii isolate LUH5552 carries the KL89 capsule biosynthesis gene cluster. Capsular polysaccharide (CPS) isolated from LUH5552 was analyzed by sugar analysis, Smith degradation, and one- and two-dimensional H and C NMR spectroscopy. The K89 CPS structure has not been seen before in A. baumannii CPS structures resolved to date and includes a 3-acetamido-3,6-dideoxy-d-galactose (d-Fucp3NAc) residue which is rare amongst A. baumannii CPS. The K89 CPS has a →3)-α-d-GalpNAc-(1 → 3)-β-d-GlcpNAc-(1→ main chain with a β-d-Glcp-(1 → 2)-β-d-Fucp3NAc-(1 → 6)-d-Glcp side branch that is α-(1 → 4) linked to d-GalpNAc. The roles of the Wzy polymerase and the four glycosyltransferases encoded in the KL89 gene cluster in the biosynthesis of the K89 CPS were assigned. Two glycosyltransferases, Gtr121 and Gtr122, link the d-Fucp3NAc to its neighboring sugars.