The conformational change upon binding with phospholipid membrane has been studied of mastoparan from wasp venom, a tetradecapeptide causing the degranulation of mast cells. The 270-MHz 1H-NMR spectra and CD spectra indicate that the mastoparan molecule takes the alpha-helical conformation in methanol solution, but a much less ordered form in aqueous solution. On binding with phospholipid membrane, the alpha-helical conformation is formed even in aqueous medium. Such a conformational change is primarily due to the interaction between the aliphatic side chains of mastoparan and the hydrophobic interior of phospholipid membrane, in contrast to the case of melittin from bee venom.