Abstract: The infection process of Botryosphaeria dothidea in the bark of poplar and the response of poplar bark to the infection were studied. By the use of phenoamonia blue,the infected and response cells can be stained blue and thus differed from the normal cells. As the fungus progressed in the periderma,they were always intercelluse of mycothirum cells,cells 46 layers around can be affected and cytoplasima condensed into protrusions.After 40 days, phenoderma formed between the necotic tissues and normal cells,and the resistant poplar have a thicker new periderm than that of the susceptible one. Above all, the resistant poplar can form a boundary of stindark tyloses in the new periderm cells which may take the role as to restructed the hyphae from continued expansion. There is no such a boundary in the wounded but uninoculated controls. Ultrastructure study by electron micrographs showed that mycelium were abounded in the susceptible cultivars and always exist between cells or in the swollen and thickened or dissolved cell walls. In the cells of resistant poplars,a kind of electron dense, hairlike,crystal deposit were formed.