The electronmicroscopy of tissues and cells has added many new findings to our concept about their structures and is bringing the great contribution to the modern cytology. Although the fine structure of oocytes was also studied by this method, the materials used, in the studies so far published, were the ones of invertebrate (1) (2) (3) (4) (20) (22) (23) and amphibia (14), and the detailed studies on the mammalian oocytes were not published yet. At the present work, the various developmental stages of oocytes are recognized within the sections of mouse ovary. Their fine structures, especially the characteristic modification of mitochondrial structure and the peculiar bodies composed of numerous vesicular components, are reportd in this paper.