The complete amino acid sequence of neurotoxin I of the North African scorpion Androctonus australis Hector collected in Algeria (area of Chellala) is described. S ‐Methylation of cysteine residues formed by reduction of disulphide bonds was found useful for their identification in sequence determination using the phenylisothiocyanate method. From the N‐terminal sequence (38 residues) established in the protein sequenator, and from the sequence of chymotryptic peptides the complete sequence of the protein has been deduced. The molecule consists of a single polypeptide chain cross‐linked by four disulphide bridges. An isoneurotoxin (I′) has been found in the venom of A. australis collected in Tunisia (area of Tozeur). It differs from neurotoxin I in the replacement of the valine residue in position 17 by an isoleucine residue.