Solitary wasps paralyze insects or spiders with stinging venom and feed the paralyzed victims to their larvae. Accordingly, the venom should contain a variety of constituents acting on nervous systems. However, only a few constituents of the venom have been chemically characterized, despite the fact that thousands of solitary wasp species are inhabiting on the earth. We report here the isolation, structure and biological effects of novel peptides in the solitary wasp venom. Eumenine mastoparan-AF (1) was isolated from the venom of the Eumenine wasp Anterhynchium flavomarginatum micado, which was responsible for the block of neurotransmission in lobster leg. The congeners 2-4 were also obtained, but they turned out to be the hydrolysate by the protease in the venom. The structure of 1 was similar to that of mastoparan (5), a mast cell degranulating peptide in the venom of a hornet wasp, and the peptide 1 was indeed as active as 5 in stimulation of histamine release from rat peritoneal mast cells. The peptide 3 was active in this assay as well, whereas the peptides 2 and 4 were inactive. As-A (6) and As-B (7) were the major peptides in the venom of the spider wasp Anoplius samariensis, of which the pentapeptide 6 blocked the neurotransmission in lobster leg. Bm-17 (8) was obtained from the venom of another spider wasp Batozonellus maculifrons, but totally inactive in the lobster leg muscle preparation.