Abstract: This paper reports the micrographic structure and nitrogenase activity of nodules in L. leucocephala at their different stages of development. Nodules changed considerably in external shape during their growth, from sphere or column to multibranch. However, all forms of nodules had the common structure,consisting of bacteroid-containing tissues, vascular tissues, meristems and cortices. The highest nitrogenase activity, as determined by acetylene reduction assay, was observed in the mature nodules; the juvenile nodules ranked the second;and the senescent nodules followed as the last. The mature nodules contained the greatest number of bacteroids. The juvenile nodules had more bacteroids than the senescent nodules.The possible mechanism of meristematic differentiation in nodules are also discussed in this paper.