Chromosomal DNAs were extracted from Clostridium butyricum strain BL6340 and Clostridium botulinum type E strain Mashike. The 6.0 Kbp fragment coding for the entire light chain (L) component and the N-terminus of heavy chain (H) component of botulinum type E toxin was obtained from each extracted DNAs after digestion with HindIII. The entire nucleotide sequences for the light chain components of these cloned genes were determined, and the derived amino acid sequences were compared to each other, and with those of botulinum type A, Cl, D, and tetanus toxins reported previously. The cleavage site of L and H components of type E toxin was presumed to be Arg-422. In a total of 422 amino acid residues of L component, 17 residues were different between butyricum and type E toxins, and all these differences were found within 200 residues of N-terminus of L component. On the contrary, five regions showing highly homologous sequences were found in L components among these six toxins, and one more region between botulinum type E and tetanus toxins.Clostridium butyricum and Clostridium baratii strains which produce botulinum type E and type F neurotoxins, respectively, were isolated from human infant botulism (7, 10). Type E neurotoxin is produced as a single polypeptide chain, and nicked by protease (trypsin) to separate into two fragments, designated light chain (L; Mr, 50,000) and heavy chain (H; Mr, 100,000), following reduction of a disulfide bond connecting these fragments. The toxin inhibits acetylcholine release from nerve endings, but its detailed mechanisms are still unknown. It is postulated that the heavy chain which consists of the C-terminus of the toxin correlated with the binding of toxin to peripheral synapses, and the light chain which consists of the N-terminus of the toxin associated with the activity to block acetylcholine release.