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The arrangement of C-tubule protofilaments was studied in basal bodies of respiratory epithelial cilia using tannic acid staining techniques. At the proximal end of the basal body, the C tubule consisted of 10 protofilaments arranged analogously to the 10 protofilaments of the B tubule. In the region of the basal foot, 5 of the C protofilaments sequentially terminated. First, the C10 and C9 filaments terminated. Then the C1 and C8 filaments terminated, and finally the C7 filament terminated leaving a curved sheet of 5 filaments C2-C6 coursing through the distal third of the basal body. This study validates previous suggestions that the C tubule is incomplete distally, and this study clearly demonstrates the extent and consistency of this incompleteness. This morphological study also raises again the question of the function of the C tubules of basal bodies in light of both the incomplete nature of the C tubule and the apparent minor role of the C tubule in providing attachment sites for the basal foot and the alar sheets.