On the basis of experim ental atudies on the spat rot disease of the fronds of the haidai, Laminaria jnponive, the following results had been obtained. 1.The disease developed more seriously in shallow water regions where water circulation waa slow, and the spread more rapidly with increase of water transparency. 2.No bacteria were iaolated from the healthy parts of the diseaaed frvuda.Bacteria isolated from the diseased parts were found to be Cram-negative bacilli without spores, and inoculation of these bacteria into healthy fronds did not produce similar diseases. 3.Anatomy of the diseased fronds showed no bacteria in tissues in the early stages of the disease; in later stages, the cells last all their pigments, only the protoplasm being left, but the epidermal cells of the ether shadowed side of the fronds remaining healthy; when the white spots developed further and eventually merged with the neighboring spots into large a hole, often a pigmentrd ring was found around the hole. On the basis of these observations, it was concluded that the spot rot disease is physiological in nature and is not caused by pathogenes, although the bacterial growth which eventually followed does worsen the disease. Therefvre the authors suggest that to prevent the disease, means must be taken to make better water circulation in the cultivation regions and to lower the cultivation depth of the haidai at the time when the Laminaria is moat easily infected with this disease.