When a large quantity of the venom of the scorpion Androctonus australis Hector is submitted to a purification procedure it is possible to purify a new toxin, toxin I", which is lethal to mouse, and present in very low quantities. The yield is 0.04%. The amino acid sequence of this newly discovered toxin only differs from toxin I of the same scorpion by an extra arginine residue at the C-terminal end provided that the sequence of toxin I, at positions 34 and 35, is in agreement with newer information provided in this paper.