Ciliary complexes termed the kinetids, contain fibres of several kinds attached to the proximal end of a basal body. One of these fibres, the long microtubular fibre running in the endoplasm ofDileptus, is of special interest for this study. The fibres when attached to oral kinetids are orientated towards the cell posterior, and are numerous in the proboscis endoplasm. The fibres anchored at locomotor kinetids are orientated towards the cell anterior and penetrate the endoplasm of the tail. The endoplasm of both proboscis and tail appears transparent when viewed in the light microscope, and is deprived of many organelles: nuclei, lipid droplets, and food vacuoles. During regeneration proboscis and tail reconstitution is simultaneous, with an increase in transparency and in the density of microtubular fibres within the regenerating region. In posterior fragments ofDileptus which contain locomotor kinetids only, oral kinetids form as an offspring of locomotor ones. During differentiation of oral structures oral kinetids rotate until their endoplasmic fibres point posteriorly. It is this rotation that supplies the cell with a posteriorly directed set of endoplasmic fibres. The possibility that the translocation of endoplasmic organelles along the microtubular fibres may be one of mechanisms in shaping cells is discussed. Since the direction of endoplasmic translocation depends upon fibre orientation, the MTOCs which govern this orientation are likely candidates to be bearers of information concerning cell shape inDileptus.