Mastoparan B, a cationic toxin, is the major peptide component in the venom of Vespa basalis. Molecular cloning of its cDNA fragment revealed that this toxin was initially synthesized as a precursor polypeptide, containing an N-terminal signal sequence, a prosequence, the mature toxin, and an appendix glycine at C-terminus. Sequence alignment between precursors of mastoparan B and melittin from honeybee venom showed a significant conservation in prosequence. Alternate positions existing in both prosequences were either proline or alanine known as the potential cleaving sites for dipeptidyl peptidase IV. Subsequently, a putative dipeptidyl peptidase IV cDNA fragment was cloned from Vespa basalis venom gland. The prosequence may possibly be removed via sequential liberation of dipeptides during the processing of mastoparan B.