New spider toxins, nephilatoxins, were purified from the venom of Joro spider, Nephila c/avara, and characterized. Nephilatoxins are structurally similar to each other and possess a 6-hydroxy-indole-3-acetyl or indole-3-acetyl moiety linked with N-termini of the polycationic part composed of basic amino acids and polyamines. The toxins induce histamine release from rat peritoneal mast cells. Some of the toxins suppress the excitatory postsynaptic potential in the lobster walking leg. In addition, the toxins show insecticidal action against wriggler of mosquitos when they were added in the water.