Root-nodules of soybean infected by Rizobium were studied in electron microscope. Cubes of nodule tissue filled with bacteroids were fixed in KMnO_4 or OsO_4, embedded in Stylene. Sections were cut with Hitachi UM-3 ultrotome with glass knives and were examined with HS-6 electron microscope. The central tissue (pink to red) of root-nodule were filled up with bacteroids. And in the host celles, bacteroids were embedded in enclosing membrane. The author considered that enclosing membrane developed from host cell membrane, although there was no direct evidence whether the membrane enclosing some bacteroids was devided from the true cell membrane or not. The nuclei of bacteroid cells were similar to each other and some-times nuclei were deformed by multiplication and oppresion of bacteroids. Occasionally, proplastids and needle-like crystals were observed in the bacteroid cells. On the other hand, endoplasmic reticulum and electron-dense areas without limiting membrane were observed in the non-infected cell of nodule. These dense areas were presumed to be lipid rich globular bodies because they were more stained with osmium solution. Common forms of bacteroids were rod-shape, but often showed various shapes in nodules. It seemed that such bacterial deformations were produced by changes of local phenomenon in the nodule tissue.