A new toxin, named prosurugatoxin because it formed surugatoxin on decomposition, was isolated from the toxicated Japanese ivory shell (Babylonia japonica). Prosurugatoxin evoked mydriasis in mice at a minimum effective intraperitoneal dose of 15 ng/g body weight, and inhibited the contractile response of isolated guinea pig ileum induced by 3×10⁻⁵ g/ml of nicotine at a concentration of 5×10⁻⁹ g/ml. In this mollusc the prosurugatoxin content was about 5 times that of neosurugatoxin. Prosurugatoxin was deduced on the basis of physical and chemical data to be des-xylopyranosyl-neosurugatoxin.