The amino acid compositions of extracts from the venom sacs of Vespa mandarinia, V. xanthoptera, V. tropica, V. analis and Vespula lewisi were analyzed. omega-Amino acids, which are inhibitory neurotransmitters, such as gamma-amino-n-butyric acid, taurine, beta-alanine and glycine, were predominant components in Vespa, but a minor component in Vespula. Glutamic acid, an aminergic excitatory neurotransmitter, was present in large quantities. Leucine, an insect autotoxin, was detected. Arginine was found in all the venoms and glutamine was also abundant. Tryptophan and histidine, precursors of serotonin and histamine, respectively, were also found. Thus, neuroactive amino acids that may exert inhibitory effects on insects neuronal axons and neuromuscular junctions were present in large amounts, and may facilitate paralysis of insect prey.