Abstract
The tomato plant, Lycopersicon esculentum Mill, was inoculated with tumor inducing strain, $A_6K_1$, of Agrobacterium tumefaciens and its produced tumors were examined with the electron microscope. A number of bacteria are usually detected in the intercellular region of the host plant, and it is observed that the host cytoplasm is readily destroyed in the region where the bacterial invasion occurred. Some of the bacteria in the host tissues are enclosed with the single unit membranes, in other locations lots of bacteroids were examined and the bacterial lysis is generally observed in those bacteroids. The bacterial movement in the tumor tissue and some peculiar relationships between the pathogens and the host plant are discussed.