Abstract: The ultrastructure of root nodules of winged bean has been examined by trans mission electron microscopy.During development winged bean root nodules undergo a series of ultrastructural changes including the release of bacteria from the infection thread, rhizobia(Rhizobium sp.)infection that released into the host cells gradually turned to N-fixation bacteroids.The bacteroids are enclosed by envelope membranes(enclosing membrane or bacteroid perimembrane).In any asynchronous there are four different cells structural forms in root nodules:(A)immature bacteroids,(B)mature bacteroids,(C)senescent bacteroids and(D)infection thread rhizobia.During the individual development of a bacterial cells, a serie, of changes in morphological character have been observed.These changes are described in detail and the significance of the structural changes and their relationship to the development of root nodules are discussed.