From a cDNA library made from telsons of scorpions of the species Androctonus australis, full-length cDNAs of about 370 nucleotides encoding precursors of toxins active on mammals or on insects have been isolated using oligonucleotide probes. Sequence analysis of the cDNAs revealed that the precursors contain signal peptides of about 20 amino acid residues. In addition, precursors of toxins active on mammals have extensions at their COOH-terminal ends: Arg or Gly-Arg. The processing steps required to generate toxins from their respective precursors are thus not identical for all of them. Southern blot analysis performed at the genomic level with a cDNA encoding toxin II suggested a single copy gene having a minimum size of 2800 base pairs. Finally, in an attempt to successfully express an animal toxin, monkey kidney COS-7 cells transfected with a plasmid harboring a cDNA encoding toxin II transiently expressed a recombinant toxin having the immunological and biological properties of toxin II.