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We raised monoclonal antibodies against homogenates of ovaries of Ciona intestinalis. We obtained an antibody named GC-1 which specifically recognized early germ cells in C. intestinalis and C. savignyi. Using GC-1 as a marker in immunoelectron microscopy, we determined the morphological sequence of early oogenesis in the ovaries of Ciona. In the stratified epithelium composing the wall of the ovarian tubes, the oocytes were identifiable at the early stages of meiotic prophase according to nuclear features such as condensed chromatin with synaptonemal complexes. GC-1 recognized these early oocytes. We found round cells with large and homogeneous nuclei clustered at the marginal end of the stratified epithelium. We identified these cells as oogonia on the basis of: (1) features of the nucleus, (2) reactivity to GC-1, and (3) early emergence in the developing ovaries. The oogonia were classified into three types: type A was large (7–9 μm in diameter) and clear, type B was intermediate in size (5–6 μm) and electron-density, and type C was small (4–5 μm) and dark. In the developing ovaries of juvenile C. intestinalis, type A oogonia appeared first (before 11 days after settlement) and types B and C followed (15 days after settlement). Thus we see that the type A is the oogenetic stem cell, type B is the proliferating oogonium, and type C is the final oogonium just before meiosis. The oocytes appeared 18 days after metamorphosis.