Previous studies by our group have demonstrated that the capsule of Bacteroides fragilis type strain NCTC 9343 consists of two chemically distinct polysaccharides, designated PS A and PS B. These polysaccharides can be isolated as an aggregate from the surface of the organism and give a complex multiprecipitin profile when they are reacted with homologous antiserum in an immunoelectrophoresis assay. Following structural analysis of PS A and PS B, we have determined that the complex precipitin profile is formed as a result of the differing electrophoretic and antigenic properties associated with each of these polymers. Presently, we have examined the capsular polysaccharides of 13 other strains of B. fragilis according to methods used for the prototype strain. The capsules of these strains were extracted, partially purified, and analyzed by immunoelectrophoresis at pH 7.3. Following reaction with homologous polyclonal antisera, each of the capsular preparations tested yielded a complex precipitin profile similar to that of the prototype strain. When reacted by immunoelectrophoresis with polyclonal antiserum to 9343 or with monoclonal antibodies to PS A and PS B, these capsular preparations appeared to be antigenically diverse; some preparations (50%) showed complete or partial cross-reaction. These results suggest that the dual polysaccharide motif seen with the prototype strain is a common feature of B. fragilis strains. In addition, the antigenic heterogeneity of B. fragilis capsular polysaccharides could be used for the development of a serological typing scheme.