A tridecapeptide with the sequence CCEICCNPACFGC has been synthesized to reproduce the active moiety of a heat stable enterotoxin from Vibrio cholerae. The proton NMR analysis indicates, for the active synthetic fragment, a rigid secondary structure stabilised by three disulfide bridges. Such a rigid peptide, suitably detoxified and activated, could be a good candidate to be used as a carrier for linear bioactive peptides or other functional groups.