1. A toxic substance possessing a strong mydriatic activity was found in the mid-gut gland of the Japanese ivory shell, Babylonia japonica, collected at Numazu, Yokohama and Kamakura. A sample from Teradomari on the Japan Sea side where an outbreak of poisoning occurred in 1957 was found to be non-toxic. 2. An assay method for the ivory shell toxin based on mydriasis of mice was established. 3. The toxin is soluble in water, slightly soluble in methanol and ethanol and insoluble in most fat-solvents. It is unstable to heat, especially in an alkaline medium, dialyzable through a cellophane membrane and not steam-distilled. The purified toxin is positive for the ninhydrin reagent, fluorescent under ultraviolet light and negative for the Dragendorff and biuret reagents.