The changes of histology and cytology in Laminaria japonica were studied by art ificial infection and microscopic observation. The results showed as follow s: 1. The endodermis was infected f irstly by alginic acid decomposing bacteria ; the exodermis , pith and epidermis were infected later. 2. The damage of endodermis was heaviest , exodermis and pith were damaged less heavily, and the effect of alginic acid decomposing bacteria on epidermis was least remarkable. 3. a lot of storage material was accumulated in endodermic cells of infected laminaria japonica compared with control; the diameter of storage particles was from 1. 5 Lm to 4. 0 Lm, and the average value was 2. 7Lm.