AbstractThis paper describes the ultrastructural phenomena occurring during the differentiation of cilia in the epidermis and trachea ofXenopus laevis. The sequence of developmental events seems to begin with thede novoformation of clusters of small, dense “procentriole precursor bodies” which are apparently transformed into procentrioles under the influence of dense amorphous masses here termed “procentriole organizers.” The procentriole so formed is a cylinder 150 mm̈ in diameter with nine single microtubules in its wall which probably give rise to the nine “triplet” microtubles in the wall of the definite centrioles (200 mm̈ in diameter). The appearance of multiple centrioles in the apical cytoplasm is correlated with disappearance of procentriole precursor bodies and procentrioles. Smooth‐surfaced vesicles seemingly produced in the well developed Golgi zones may contribute to the development of the ciliary membrane. Following their formation in the supranuclear cytoplasm, the centrioles or presumptive basal bodies align with their longitudinal axes perpendicular to the apical plasmalemma. A ciliary shaft then appears distal to each basal body and simultaneously small, dense bodies like those termed “procentriole precursor bodies” reappear. At this time we call the dense bodies, “axonemal precursor bodies,” because they probably provide material for the synthesis of new axial microtubules at the tip end of the growing cilium. Centriolar rootlets develop during centriolar alignment and ciliary shaft formation from a distinct, low electron‐op