Abstract: Calli of poplar susceptible and resistant to poplar blister canker disease were exposed to the toxin produced by a pathogenic isolate of Dothiorella gregaria at two concentrations. All the cells exposed to toxin showed a series of sub-cellular ultra-structural changes such as distorted and collapsed cell walls and plasmolysis;ruptured plasma membranes;disrupted outer-membranes of mitochondria, some of them even broke down and became granular or hollowed;membrane of nucleolus dilated and part of it decomposed. Cells from the susceptible poplar displayed mitochondria and cytoplasmic disruption. Plasma membranes appear to be the first organs to be attacked seriously. Cells from the untreated tissue exhibited typical internal composition associated with healthy, actively growing cells. The differences between the susceptible and resistant poplar, which were exposed to high level and low level of toxin concentration are that: at the same time after exposed to toxin, the ultrastructure of susceptible poplar were more serious disturbed than that of the resistant one, and the same influences showed in the high level of toxin concentration.