A large number of investigations which have been carried out to elucidate the structure of the basic components of the ciliary shaft, have been reported by FAWCETT and PORTER (1954), GIBBONS (1961), BRICHTMAN and PALAY (1963) and others. In a majority of organisms, the basic components of the ciliary shaft are two central fibers surrounded by a ring of nine outer doublet fibers, these fibers usually having the appearance of hollow tubes. As the structure of the ciliary shaft is approximately the same in both protozoa and metazoa, description in this paper will be confined to that on the ciliary basal body complex, which has been treated of in few previous works. It seems likely that each component and structure of the ciliary basal body complex vary greatly among different species of animals or different tissues, and the functional differences should accompanied the structural differences. If, however, among cilia of several kinds which have different functions, there is a special difference proper to the structure of their basal body complex and none in the structure of their shaft, the motive or starting point of ciliary movement should be located in the structure of the basal body complex. In this study, the oviduct epithelium of the mouse has been examined by electron microscopy.Tissue blocks from the oviduct epithelium of mature mouse were fixed for 2 hours in a 1% solution of OsO4 (buffered at pH7.4 with s-collidin). Other pieces of the tissue were fixed for 2 hours in a 6% solution of glutaraldehyde and then postfixed for 2 hours in a 1% solution of OsO4 (buffered at pH7.4 with SORENSEN's phosphate buffer). The tissue blocks were then dehydrated in ascending concentrations of ethanol, and embedded in araldite. Sections were stained with a solution of uranyl acetate and examined by the Hitachi HU-11A type electron microscope.The construction of the ciliary basal body complex in the oviduct epithelium of the mouse resembled that in the tracheal epithelium of the mouse (UEKI, 1966), but differed from those in the palatine epithelium of the frog (UEKI, 1965), i. e. in the former neither rootlet nor basal plate, but the“circular basal band”(named by UEKI, 1966) as well as the special composition of the“principal band foot”(named by UEKI, 1965) have been observed.The fibers which constitute the principal basal foot are homogeneous and relatively low in electron density and converged in a form of Japanese hair pencil; they curve outwardly leaving the longitudinal axis of the basal body, showing anti-clockwise spiral course. Thus, the bundle of fibers constituting the principal basal foot is twisted anti-clockwise.Nine transitional fibers which start from the distal ends of nine outermost subfibers in the basal body also take the course of sinistral spiral.Nine links in the neck are each nearly straight, but as a whole they form a sinistral spiral radiation.In a series of these anti-clockwise spiral formations are involved also the nine doublet outer fibers of the shaft. Accordingly, it is considered that the ciliary movement of the oviduct cilia of the mouse is not a simple to-and-fro movement in a plane, but a spiral rotation.