A novel protein toxin, named Clostridium chauvoei toxin A (CctA) that belongs to the family of ß-barrel pore forming toxins of the leucocidin superfamily of bacterial toxins was discovered by whole genome sequence analysis. The corresponding gene cctA and the secreted toxin CctA was found in all strains of C. chauvoei analyzed, isolated from various geographical areas over the globe during the last 50 years, but not in other pathogenic Clostridium species. Native CctA and recombinant rCctA produced in Escherichia coli are highly cytotoxic towards Embryonic Calf Nasal Epithelial (ECaNEp) cells and have high haemolytic activity against sheep and bovine erythrocytes in standard haemolysis assays. Polyclonal anti-rCctA rabbit antibodies fully neutralize the cytotoxic and haemolytic activity, not only of rCctA but also of supernatants from cultures of the various C. chauvoei strains, indicating that CctA is the main cytotoxic and haemolytic substance secreted by C. chauvoei. Using a standard vaccine release procedure, we demonstrated that vaccination of guinea pigs with recombinant CctA in form of a fusion protein with the B subunit of the E. coli heat labile toxin as a peptide adjuvant (rCctA::LTB) protected the animals against challenge with spores of virulent C. chauvoei. This indicates that CctA is the major virulence factor of C. chauvoei and a valuable antigen in vaccines against blackleg.