Abstract: This paper deals with a periodical observation on the nodulation of locust (Robinia pseudoaca-cia). The preparations of successive paraffin section are observed as well. The cortical parenchyma cells along the path of bacterial invasion beging to divide and the invaded cells increase in size as the bacteroids appear. The increased meristematic activity and cell enlargement of cortical cells result in the formation of the nodule which grows outward from the root cortex. At the same time differentiation of vascular tissues,both xylem and phloem, takes place in the nodule. The vascular tissues of the nodule are connected directly with those of the root. The shape of the nodule is governed by the extent and location of the meristem, which is the site of active cell division and tissue differentiation.