The leaves of the wild potato, 5. demissum, which contain about 0.5% of demissin (solanin d, C50H83-O29N), were not eaten by the larvae of the Colorado beetle, Leptinotarsa decemlineata, and also leaves of S. tuberosum were almost never eaten when they were covered with a layer of gelatin containing demissin in a concn. of 1%, or when a demissin chloral hydrate soln. (pH 5, concn. 5%) or demissin in a concn. of 0.3-0.4% had been infiltrated into them. Larvae to which these leaves had been offered did not eat anything, or ate only quite insufficient amts.; they remained as small as those to which no food had been offered, and usually died within a week. Neither the artificially increased contents of 1.3% of solanind (which is normally present in the leaves of the cultivated potato in a concn. of 0.01%), nor a layer of pure gelatin, nor the infiltration of dist. water or HCl-ammonia protected the leaves of S. tuberosum against being eaten by the larvae of the Colorado beetle.