A new marine toxine, surugatoxin was isolated from a carnivorous gastropod, Babylonia japonica captured in Suruga Bay. This paper deals with isolation and chemical structure of surugatoxin. Surugatoxin evokes a mydriasis in mice like atropine, at a minimum dose of 0.1μg/g body weight, and is quite unstable against acid, alkaline and heat. Surugatoxin has following properties; colorless prisms, mp>300℃, C_<25>H_<26>N_5O_<13>Br, UV λ^<H_2O>_<max> mμ(ε) 276 (14,000), IR υ^<kBr>_<max> cm^<-1> 3200, 1740 (sh), 1695, 1640. Degradation of surugatoxin with 1N-hydrochloric acid or 2N-ammonium hydroxide at 100C afforded one mole ratio of myo-inositol. The chemical structure of surugatoxin was determined as (II) by X ray analysis.