1. The symptom of a new Bacterial disease, the Nankwabyo, of Gypsy-moth caterpillars (Lymanthria Dispar, L.) resembles very much to that of the Nankwabyo, a kind of flacherie, of silkworms. The disease generally appears at the time, when the worm has reached its fourth or fifth moulting skin. 2. The disease is very infectious and is widely spread in our couutry. As a natural enemy to the Gypsy-moth caterpillar, it is togather with a parasiteic Hemiptera plaing the most importsnt role in the checking of the spread of this injurious insert in Japan, where as a consequence it is seldom to see such a devastation from the ravages of the worm as in America. 3. The disease is caused by a new species of bacteria, -Bacillus Disparis Hori et Bokura. It is medium-sized, rod-shapid, 1-1.3 0.7-0.8 microns, with 6 to 8 peritri chiate flagella. Endspores not formed. By the Gram's method, not decolorized. Konjak-media not liquified. Fermentation is readily set up in dextrose, laevulese, saccharose, maltose and galactose. Optimum temperature is 30° to 32°C. When fed to Gypsy-moth caterpillars they die readily in one to eight days; but it is ineff ctive when fed to silk-worms as well as to Anomala rufocuprea Motsch., Pieris rapae L, Aphis sp. on a lily and Hemerophla atrilineata, Butl. on mulberry. 4. To make a practical use of this bacteria for the extermination of Gypsy-moth caterpillars, spray on plants the worms are feeding, sterilizel Water in which the bouillon or soy-bean cake decoction culture of Bacillus Disparis is thoroughly mixed up. or it is more economical and convenient to make use of thk body fluid of the diseased or dead caterpillars instead of the pure cultures for the spraying purpose.