AbstractThymocytes of young rats were studied with the electron microscope to provide a base line for a comparative study of the fine structure of various types of lymphocytes. Fixation with OsO4or glutaraldehyde, Epon embedding, and heavy metal staining were the principal techniques employed. Small thymocytes, with nuclei from 3.5 to 4.5 μ in diameter are in the vast majority. Our results, which in general confirm previous reports, show the thymocytes to be primitive, or undifferentiated in structure. Certain exceptions and additions were noted, however. There are no protoplasmic bridges between thymocytes and epithelial cells and no clear evidence of epithelial‐thymocyte transformations. The nucleus contains typical pores and in many cases a clearly defined finely granular nucleolus, although the latter is not evident in most of the smallest cells. Cytoplasmic fragments appear to be separating from thymocytes by a process similar to the separation of platelets from megakaryocytes. The existence of a special band of cytoplasm beneath the plasma membrane devoid of formed elements is described for the thymocytes. The possibility of the common occurrence of this zone in other cells, and its possible significance, are discuss