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The histologic and cytologic structure of the boring organ of Thais lapillus has been studied with both light and electron microscopy. The organ evaginates from the foot and has a dome-like form. It has a spongy center which contain nerves, muscles and blood vessels, these originate in the foot and continue into the boring organ via a short stalk. The organ is invested externally by a continuous simple epithelium constituted of extremely elongated cells (150 to 300 mu). There are three radial zones from the basal to the apical aspect of the cells : basally the cells are overlapping, here are located the nuclei, the Golgi apparatus, the rough endoplasmic reticulum and secretory granules; organelles which in their character suggest a secretory function for these cells. In the middle zone cells are separated into "groups" by large spaces occupied by blood sinuses; at this level are located numerous elongated mitochondria with closely apposed, angulated cristae. In addition there are microtubules arranged parallel to the main cellular axis, whose role may be to facilitate rapid transit of secretory granules from the basal to the apical zone of the epithelium. In the apical zone the cells expand in tightly interdigitating folds, having a height of 2 mu, which are joined together by junctions of various types. This zone is the only junctional area between the epithelial cells of this expanding and contracting boring organ. A network of microfilaments which penetrates the cellular folds at this level, probably plays an important role in the invagination of the organ. Finally at the apex, the epithelial cells have a high "brush border" of long microvilli whose plasmalemma is coated internally by particulate subunits of 30 A. This peculiarity may be an other fact to support the hypothesis that hydrogen ions are emited by these cells, by the action of carbonic anhydrase located perhaps in these subunits.