The archaea Methanococcus maripaludis strain Mm900 produces flagella that are glycosylated with an N-linked tetrasaccharide. Mass spectrometric analysis of flagellar tryptic peptides identified a number of tryptic glycopeptides carrying a glycan of mass 1036.4Da, and fragmentation of the glycan oxonium ion indicated that the glycan was a tetrasaccharide. The glycan was purified, following extensive pronase digestion of flagellar filaments, by size-exclusion and anion-exchange chromatography. NMR spectroscopy revealed that the glycan had the following structure: Sug-4-beta-ManNAc3NAmA6Thr-4-beta-GlcNAc3NAcA-3-beta-GalNAc-Asn where Sug is a novel monosaccharide unit, (5S)-2-acetamido-2,4-dideoxy-5-O-methyl-alpha-l-erythro-hexos-5-ulo-1,5-pyranose. This oligosaccharide has significant similarity to the oligosaccharide that was found previously in Methanococcus voltae.