Electron microscopic observation was applied to both twenty cases of normal mucosa and eighteen cases of squamous cell carcinoma of the oral cavity and their exfoliative cytologic findings were investigated in comparison with the histologic findings. Ⅰ. TISSUE CELLS Desmosome, which are regarded as a universal structure of cohesion of epithelial cells, was much poorer in carcinoma cells than in normal ones and intercellular spaces displayed a marked variation in width in carcinoma tissue cells. Thus intercellular cohesion was morphologically very poor and loose in carcinoma. In both cells an intercellular border was distinct and no syncytium was found to be formed. Tonofilaments, which can be distinctly found in normal cells, could not be noticed and a higher development of endoplasmic reticulum and mitochondria was shown in carcinoma cells. However, the mitochondria of carcinoma cells was remarkable in degenerative findings and was larger in size than that of normal cells. There was a strong resemblance between basal cells of the normal tissue and carcinoma cells in an indefinite nuclear shap and a decrease of cytoplasmic: nuclear ratio. In carcinoma cells a fine structure of their nuclear membrane was well-preserved and their nucleoli could be remarkably noticed. Ⅱ. EXFOLATED CELLS In normal cells spinous processes of almost definite size and shape were extending regularly around them, while in carcinoma cells poorer processes of various shape and size were extending irregularly around them or sometimes they were missing. Mechanical destruction was particularly marked in the area of peripheral cytoplasm of normal cells of the deeper layer under the superficial spinous cell layer probably because of a tight intercellular cohesion. Generally speaking, findings of tonofilament, endoplasmic reticulum and mitochondria corresponded to those of tissue cells in both normal and carcinoma cells, while a slightly severer degeneration of intra-cytoplasmic organelles in exfoliated carcinoma cells could be noticed in comparison with carcinoma tissue cells. A fine structure was well-preserved in nuclei of carcinoma cells, but disintegration and disappearance of the nuclear membrane could be noticed in severely degenerated carcinoma cells and no thickening of the nuclear membrane could be found. From these results obtained the cytologic findings of the smear in Papanicolaou staining and electron microscopy were compared and discussed.