Abstract:The relation between the endoplasmic reticulum and peribacteroid membranes during the development of infected cells of Chinese soybean (Glycine max L. cv. Harvest 11) root nodules by transmission electron microscopy was observed. After the host cells are infected by bacteria, the ultrastructures of the infected cells appear to have many changes, such as that their cytoplasm becomes thicker, the vacuoles decrease in size and organelles rapidly increase in number, among these organelle changes are more obvious than the others. However, changes of endoplasmic reticulum is mostly striking. It is not only increases greatly in number but often swells and forms wider inter-spaces. The swelling of endoplasmic reticulum is especially conspicuous at its ends and often form various vesicles. Sometimes, the front part of the endoplasmic reticulum also forms a gourd-shaped structure, which together with the vesicles usually contain fibrillar material. After they are released from the endoplasmic reticulum to the host cytoplasm, they continuously move towards neighbouring bacteria and close to the peribacteroid membranes. The gourd-shaped structures always locate near but never fuse with the peribacteroid membranes. However, the vesicles can do that and form a kind of papillae, often containing fibrillar material, on the peri bacteroid membranes. These papillae and their fibrillar material gradually disappear whilst the membrane of the vesicle derived from endoplasmic reticulum becomes one part of the peribacteroid membrane by way of fusing with the latter to form a papilla on it.