Haemophilus parasuis is a Gram-negative coccobacillus and the etiologic agent of Glässer’s disease (polyserositis) in young pigs and pneumonia in adults. There are 15 recognized serotypes of H. parasuis, but the serotype-specific antigen has not been purified or characterized. H. parasuis is also proposed to be encapsulated, but the capsular antigen has also not been purified or characterized, and the presence of capsule appears to be variable among strains. We have purified the capsular polysaccharide (CP) of H. parasuis serotypes 4, 5, 9, 13, and 14 by Cetavlon precipitation, enzyme digestion, phenol extraction, and gel filtration chromatography. The CP was a heterogeneous, high molecular size polysaccharide, as determined by gel filtration, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Alcian Blue/silver staining. The chemical composition of the CP is being analyzed. The CP was primarily present and isolated from autoclaved whole cells after 24 h of culture, but was present primarily in culture supernatant following 72 h of culture, indicating the CP is attached to the cells, but shed during late stationary phase. Mouse antiserum to the purified CP of serotype 5 reacted with serotype 5 cells and purified CP by ELISA, immunoblotting, and other immunological assays, but not with other H. parasuis serotypes. Following daily passage of H. parasuis on agar medium the expression of CP diminished. Comparative analysis of the serotype 5 genome sequences with capsule export/synthesis regions of Escherichia coli capsule groups I-IV identified a putative region that may encode for the proteins responsible for CP export and synthesis, and was most similar to capsule group I, rather than groups II/III capsules, which are common to other members of the Pasteurellaceae. Therefore, H. parasuis expresses a typical CP that is responsible for serotype-specificity, though genetically distinct from other family members, and likely enhances virulence of the bacterium.