Abstract:Ultrastructural changes of the infected cells and the bacteria in Onobrychis viciaefolia root nodules were observed by transmission electron microscopy. Onobrychis viciaefolia root nodules during their development possessed main features as follows: 1. In early developmental stages of the infected cells, there were often many small vacuoles in their cytoplasm. 2. Comparing with the development of the infected cells, except some bacteria still maintained rod-shaped, other bacteria gradually become to dumbbell-shaped, even short-chMn-shaped, lotus-root-shaped or other shapes. These bacteria had no obvious nucleoid zones and fibrillar-shaped nucleoid material and polyphosphate granules, however, there was a kind of especial inclusion in their cytoplasm. 3. The perilyacteroid membranes of the bacteria often occurred the fusion each other.