The structure for the carbohydrate moiety of the lipooligosaccharide (LOS) from the commensal Haemophilus somnus strain 129Pt was elucidated. The structure of the core oligosaccharide and O-deacylated LOS was established by monosaccharide and methylation analyses, NMR spectroscopy and mass spectrometry. The following structure for the major fully extended carbohydrate glycoform of the LOS was determined on the basis of the combined data from these experiments. [Carbohydrate structure: see text]. In the structure Kdo is 3-deoxy-D-manno-octulosonic acid, Hep is L-glycero-D-manno-heptose and PEtn is phosphoethanolamine. Minor amounts of glycoforms containing nonstoichiometric substituents glycine and phosphate at the distal heptose residue were also identified.