The morphology and distribution of microtubules in the ameloblast of the rat incisor were examined. Two types of microtubules distribution were observed: isolated single tubules and multiple tubules. The latter formed bundles of tubules and filaments or sheets of filaments. The thickness of the filament or sheet was almost the same as that of the wall of the microtubule. The filament or sheet as well as the microtubule were negatively stained when treated with tannic acid. The microtubules of the single type were small in number at the undifferentiated stage of the cell. They increased in number in the distal portion of the cell during differentiation, and their number reached a peak in the proximal portion of Tomes' process at the secretory stage of the cell. On the other hand, the multiple microtubules showed a different distribution in the secretory stage of the cell, i.e., they were mostly distributed between the Golgi region and the distal terminal web.