Abstract: Leaf explants of tobacco were cultured on MS medium supplemented with 2 mg/ l NAA and 0.5 mg/l BA for induction of callus formation, or supplemented with 2 mg/l BA for bud formation. Histocytological observations on callus and bud formation were carried out. Three days after cultivation, mesophyll cells enlarged, the nuclei became more apparent and dark stained, and starch accumulated in the cells. Cell divisions began in the mesophyll cells at the cut ends, in the palisade cells near the vascular bundles and in the vascular parenchyma. Mitotic activity then spreaded over tbc explants, and was most active at the edges of leaf explants. Regular rows of cells appeared as a result of series of transverse divisions in the palisade. The number of chloroplast in the mesophyll cells decreased and degenerated gradually. A number of meristemoids ware initiated in the cultured leaf explants after 7 days of cultivation. They were originated from two kinds of tissues, the mesophyll and vascular bundle, including the phloem parenchyma and vascular sheath. On the medium with NAA and BA, callus formation was induced with vigorous divisions, whereas bud primordia were differentiated from the meristomoids on the medimn with 2 mg/l BA. The buds were developed from both the superficial meristemoids and the meristematic regions deep within the callused leaf explants. The accumulated starch in the cells gradually disappeared as bud formation proceeded.