A primary cilium often projected from the Golgi region of pancreatic A, B and D cells. The proximal portion of the cilium was found surrounded by a tubular invagination of the plasmalemma, and then the cilium extended into the intercellular canaliculus. The most proximal portion of the ciliary membrane exhibited periodical densities which might correspond to the ciliary necklace. The axoneme of the cilium was basically of the 9 + 0 pattern, i.e., nine peripheral doublets and no central singlet, though it was modified along the length of the cilium. Although a few appendages were projected from each doublet, it was difficult to identify dynein arms and nexin links. At the most proximal portion of the cilium, a "champagne-glass" structure connected each doublet with the ciliary membrane. The distal and proximal centrioles of the diplosome were connected to each other by a striated band. The proximal centriole, which served as a basal body, had accessory structures, such as alar sheets, basal feet and rootlets. Frequent projections of the primary cilium and its elaborate structure suggest that the cilium is not an aberrant structure but rather one which plays a certain role in the islet cell function.