Vibrio vulnificus produces a capsular polysaccharide (CPS) that is essential for virulence. CPS from V. vulnificus clinical strain M06-24 has been purified and the structure determined. In preliminary screening with antisera raised to M06-24 CPS, 4 (19%) of 21 clinical isolates (including M06-24), but none of 67 environmental V. vulnificus isolates, agglutinated with antiM06- 24 antisera (P = .003). CPS was isolated from a subset of 12 clinical and 7 environmental isolates and analyzed by high-performance anion-exchange chromatography and one-dimensional nuclear magnetic resonance. M06-24 and 1 other serologically positive strain had identical CPS structures; the other 2 serologically positive strains had substitutions in two offour sugar residues. Thirteen other capsular types were identified among the remaining 15 strains from which CPS was extracted.