Abstract: By means of electron microscopy, the ultra-thin sections of the effective nodules of Glycine max, Pisum sativum, Phaseolus vulgaris and Sesbania cannabia were investigated. In this paper the multiple forms of the infection thread, the various structual changes in capsule and cell wall of rhizobia before and after their releasing into the cytoplasm of the host cell, the rhizobia entering into the vacuoles of the host cells and the bacteriod formation in the vacuoles were observed. Accordingly, the possibility of two different ways of the enclosing membrane formation of rhizobia was observed. The relationship between the structure and function of the rhizobia and host cells during nodulation is also discussed.