Constipation and diarrhea are common in autistic patients. Antibiotic treatment against bacteria appears to partially alleviate autistic-related symptoms. The bacterium Clostridium bolteae has been shown to be overabundant in the intestinal tract of autistic children suffering from gastric intestinal ailments, and as such is an organism that could potentially aggravate gastrointestinal symptoms. Investigation of the cell-wall polysaccharides of C. bolteae was employed in order to evaluate their structure and immunogenicity. Exploration revealed that C. bolteae produces a conserved specific capsular polysaccharide comprised of rhamnose and mannose units: [->3)-α-D-Manp-(1->4)-β-D-Rhap-(1->], which is immunogenic in rabbits. This is the first described immunogen of C. bolteae and indicates the prospect of using this polysaccharide as a vaccine to reduce or prevent colonization of the intestinal tract in autistic patients, and as a diagnostic marker for rapid detection. This diagnostic target can be used in a multivalent vaccine, which may potentially include Sutterella and Desulfovibrio.