The cell inclusions of Nicotiana tabacum L. (Hatano variety) affected with the mosaic disease were observed. X-bodies and other cell inclusions were found in all tissues of mosaic plants. Namely, they occured in all leaf tissues (epidermis, guard-cells, hairs, palisade and spongy parenchyms), in stems (with hairs), in roots (except root hairs, root caps, and calyptrogens) and in flowers (hairs, calyxes, corollas and anthers; rarely microsporogenous cells, pollen mother cells, pollens and pollen-tubes). Striate materials, raphids, crystalline plates and amorphous bodies were also found in the tissues in which X-bodies occured. Anther-tissues contained abundant crystals of calcium oxalate, and root tissues bipyramidic, rhombohedral or hexahedral crystals. Inoculation was done by means of rubbing over the leaf surface with cloth soaked in expressed juice of diseased leaves. In the leaf tissues, X-bodies and other cell inclusions were observed about 1 week after the inoculation. In the cases of root tissues, they occured within a little shorter periods. However, it seems that the period of the occurrence of cell inclusions varies according to the internal and external factors. Tracing the process of the formation of X-bodies, the cytoplasm, mostly neighbouring the nucleus, becomes denser in its appearance, and then its margin becomes obvious, and at last the body is filled with granular constitution containing vacuoles. According to the present observations, the X-bodies must be considered as being derived from the cytoplasm. Striate materials also seems to be the stratified accumulation of the crystalline products of the cytoplasm. Raphids and crystalline plates may be the crystals produced from cytoplasm. The amorphous bodies are also supposed to be the products of the cytoplasm.