Mastoparans, a group of amphiphilic tetradecapeptides, are the major peptides in social wasp venoms and possess a variety of biological activities. Here we report the first crystal structure of mastoparan from Polistes jadwagae (MP-PJ) at 1.2 angstrom resolution. The crystals beloniz to the space group P2(1) with eight molecules in an asymmetric unit. In contrast to the previous observations that the a-helical conformation only exists in the membrane-bound state of mastoparans, all of the MP-PJ molecules are in possession of the a-helical conformation even in the absence of trifluorethanol or detergents in the crystallization system. The high-resolution structure enables us to compare the conformation differences of MP-PJ with NMR results of other mastoparans. Together with biochemical results, we propose that the interactions between mastoparan molecules play an important role in forming the a-helical conformation, which is highly related to their biological activities. (c) 2007 Elsevier Inc. All rights reserved.