Abstract The carbohydrate backbone of the Hafnia alvei strain Y166/91 lipopolysaccharide (LPS) was isolated by mild acid hydrolysis followed by gel permeation chromatography and studied by NMR spectroscopy and methylation analysis. Treatment with periodate and hypoiodite gave a modified polysaccharide which was also characterised. It was concluded that the polysaccharide has the following structure with two distinct regions, which are connected. The chain length parameters m and n were not determined but the ratio m/n is approximately 2. [→ 3)-β- d-Gal p-(1 → 3)-α- d-Gal p-(1 → 3)-β- d-Gal p-(1 → 3)-β- d-Gal f-(1 →] n From the present data it is not possible to determine whether it is the Gal p-Gal p chain or the Gal p-Gal f chain that is connected to the core. The structure found here is identical to that suggested for the O-specific polysaccharide chain of Klebsiella pneumoniae O1K2 (NCTC 5055) LPS [O. Kol, J.-M. Wieruszeski, G. Strecker, J. Montreuil, and B. Fournet, Carbohydr. Res., 217 (1991) 117–125; O. Kol, J.-M. Wieruszeski, G. Strecker, B. Fournet, R. Zalisz, and P. Smets, Carbohydr. Res., 236 (1992) 339–344].