Since the initiation of haidai (Laminaria japonica) cultivation on large scale in china,several kinds of diseases have been identified, among which the most serious and most commonare the "Green Rot", "White Rot" and "Spot Rot" diseases of the thalli. A preliminary st-udy on the pathological anatomy of the diseased thalli was made and the following resultswere obtained: 1. In the parts of the thalli diseased with "White Rot", the cells were empty, havingpractically lost all their protoplasm and with no or very few chromatophores. In those disea-sed with "Green Rot", the cells had very scanty protoplasm and their chromatophores were inthe process of disorganization, appearing greenish instead of brownish as in the normal cells.No bacteria were found in the cells in the early stages of both these diseases. There fore weagree with the earlier authors that these two diseases are not caused by bacterial infection. 2. In the parts of thalli diseased with "Spot Rot", small white spots were first formed,which later grew larger and finally became large holes. Examination of the white spots re-vealed that the protoplasm and chromatophores were absent in cells, and the holes were eachsurrounded by a distinct, deeply colored ring of cells. It is our opinion that this disease, likethe above two, is not caused by bacterial infection, although in the later stages of the diseasebacterial growth may help its spread.