A new arthropod selective toxin was purified from the venom of the Venezuelan scorpion Tityus discrepans, and its amino acid sequence, cDNA clone and biological activity are reported here. The amino acid sequence of this peptide, named ardiscretin (from arthropod toxin of T. discrepans) was completed by Edman degradation and mass spectrometry. It is a single polypeptide composed by 61 amino acids with an amidated cysteine residue at the C-terminal end, closely packed by four disulfide bridges. The atomic mass unit (a.m.u.) experimentally determined was 7103.8 a.m.u. This peptide was shown to be specific for invertebrates (crickets, triatomides, crabs and squids), but non-toxic to mice, at the dose assayed. Ardiscretin inhibits the Na(+)-currents of squid giant axons in an apparent irreversible manner, whose inhibitory effect is reached at 30 microM toxin concentration. Sequence comparison showed that it is phylogenetically closely related to insect-specific scorpion toxins. Ardiscretin produced a small depolarization and induced repetitive firing in squid axons resembling those of DDT [1,1'(p-chlorobenzyl)2-tricloretane] in its ability to slow down action potential, to induce repetitive firing, and in that the concentration required for any effect in squid axon is rather high.