A novel inhibitor of voltage-gated K+ channels has been purified to homogeneity from the venom of the black scorpion Orthoehirus scrobiculosus. This toxin, named OsK2, has been characterized as a 28-residue peptide, containing six conserved cysteine residues and was shown to be a potent and selective blocker of Kv1.2 channels (K-d=97 nM). OsK2 is the second member of the 13th subfamily of short-chain K+ channel-blocking peptides known thus far and is therefore called alpha -KTx 13.2.