The Japanese ivory shell (Babylonia japonica) is occasionally poisonous, and ingestion of toxic specimens causes intoxication characterized by symptoms such as amblyopia, mydriasis, thirst, numbness of the lips, speech disorders, constipation, and dysuria. Previous studies isolated surugatoxin from toxic ivory shells, but it exhibited no toxicity after purification by high performance liquid chromatography (HPLC). In this study, neosurugatoxin, the causative agent of the intoxication, was isolated from 20 kg of the shellfish through six steps of chromatography, yielding 4 mg of the principle. This paper details the isolation process of neosurugatoxin and reports some of its biological activities, including mydriasis-inducing activity in mice (minimum effective dose of 3 ng/g) and anti-nicotinic activity (inhibiting the contractile response of isolated guinea pig ileum to 3×10⁻⁵ g/ml of nicotine at a concentration of 1×10⁻⁹ g/ml). Additionally, neosurugatoxin is extremely unstable in alkaline medium and fairly heat-labile.