The structure of the antigenic O-chain polysaccharide of Flavobacterium columnareATCC 43622, a Gram-negative bacteriumthat causes columnaris disease inwarmwater fish, was determined by high-field 1D and 2D NMR techniques, MS, and chemical analyses. The O-chain was shown to be an unbranched linear polymer of a trisaccharide repeat-ing unit composed of 2-acetamido-2-deoxy-D-glucuronic acid (D-GlcNAcA),