Analysis of mRNA transcribed off the gene encoding the depressant insect selective neurotoxin (LqhIT2) of the scorpion Leiurus quinquestriatus hebraues revealed a transcript about 400 nucleotides long and two putative precursors of about 4.4 and 3.5 kb long. Poly(A)+ RNA, purified from the venom-gland segments of the scorpion, was reverse transcribed into cDNA and a cDNA library was constructed. Two closely related cDNA clones were isolated from the library by probing with a partial cDNA clone encoding the depressant insect toxin of the scorption Buthotus judaicus. Sequence analysis of the two LqhIT2 cDNA clones revealed an identical 63 bp leader sequence, a 183 bp sequence encoding the mature polypeptide, and 9 bp encoding Gly-Lys-Lys prior to the termination codon. The two clones are distinguished by a single base substitution at the codon corresponding to amino acid residue 16 of the mature toxin, and at the non-coding region preceding the poly(A) tail. These findings elucidate the processing steps in the maturation of LqhIT2 and indicate genetic polymorphism among the genes encoding this toxin.