Centriolar pairs of ameloblasts were examined in relation to cell differentiation at the growing end of rat incisors. In their undifferentiated phase, including many mitotic cells, most of the centrioles in the ameloblasts were located lateral to the nuclei; the pairs were closely associated to each other. During the differentiating phase, the centriolar pairs moved more distally, always accompanied by the Golgi apparatus; they often became sepa-rated from each other. In the early secretory phase, and also in the following secretory phase, the pairs were usually observed distally, often away from the nuclei, and were sepa-rated from each other more frequently than in the previous differentiating phase. Through-out these phases of the ameloblast differentiation, one centriole of the pair was invariably ciliated and the Golgi apparatus was commonly associated with the centriolar pair. Such movement and separation of the ciliated centrioles is considered to be involved in the mor-phological and functional differentiation of the cells.