The process of oogenesis in Ciona savignyi was studied at the ultrastructural level to understand the development of the accessory cells and the relationship of the oocyte and its accessory cells to the germinal epithelium. There are two kinds of small cells, dark cells and clear cells, adjacent to the oocyte in the germinal epithelium. The previtellogenic ovarian follicles can be classified into four stages on the basis of accessory cell differentiation and chorion formation. At stage 1, the oocyte is enclosed by the primary follicular cells and the outer follicular cells, which resemble the dark cells and the clear cells, respectively. Late in this stage, the chorion rudiment and microvilli-like projections appear first at the area of the oocyte adjacent to the germinal epithelium. The chorion rudiment is distributed as patches on the oocyte surface and divides the primary follicular cells into inner and outer groups at stage 2. The rudiment covers the oocyte entirely, and the inner group of the primary follicular cells differentiates into the test cells, whereas the outer group becomes the inner follicular cells at stage 3. These cells contain much rough endoplasmic reticulum and electron-dense granules. At stages 2 and 3, subsurface cisterns are frequently observed at the oocyte surface facing the primary follicular cells or the test cells. The dense granules of the test cells become larger, whereas those of the inner follicular cells appear to become vacuolated at stage 4.