Using high-performance liquid chromatography Tst26, a novel potassium channel blocker peptide, was purified from the venom of the Brazilian scorpion Tityus stigmurus. Its primary structure was determined by means of automatic Edman degradation and mass spectrometry analysis. The peptide is composed of 37 amino acid residues and tightly folded through three disulfide bridges, similar to other K(+) channel blocking peptides purified from scorpion venoms. It contains the "essential dyad" for K(+) channel recognition comprised of a lysine at position 27 and a tyrosine at position 36. Electrophysiological assays revealed that Tst26 blocked hKv1.2 and hKv1.3 channels with high affinity (K(d)=1.9 nM and 10.7 nM, respectively) while it did not affect several other ion channels (mKv1.1, hKv1.4, hKv1.5, hERG, hIKCa1, hBK, hNav1.5) tested at 10 nM concentration. The voltage-dependent steady-state parameters of K(+) channel gating were unaffected by the toxin in both channels, but due to the fast association and dissociation kinetics Tst26 slowed the rate of inactivation of Kv1.3 channels. Based on the primary structure, the systematic nomenclature proposed for this peptide is alpha-KTx 4.6.