The ultrastructure of the normal thymus of the young chicken (Gallus domesticus) is described. Four main cell types, lymphoid cells, epithelial cells, macrophages and myoid cells, can be distinguished. The lymphocytes are more numerous in the cortex than in the medulla, and are quantitatively the most important component of the thymus. The epithelial cells vary greatly in morphology. Reticular epithelial cells, which have long cytoplasmic processes connected by desmosomes, and which appear to afford a supporting network for the free cells of the thymus, are present in the cortex and medulla. Undifferentiated epithelial cells are present in the medulla and cortico-medullary regions and have few intracytoplasmic fibrils or desmosomes. Cystic epithelial cells, showing intercellular and intracellular cyst formation are frequent in the medulla. Also present in the medulla are squamous epithelial cells which contain many intracytoplasmic fibrils and have numerous desmosomes, and which are involved in the formation of Hassall's corpuscles. Macrophages are present in moderate numbers in the cortex and medulla, and immature and fully developed myoid cells are common in the medulla. Other cell types present include granule-containing cells with desmosomes, large pale cells with few cytoplasmic organelles, mast cells, plasma cells, red blood corpuscles and cells of the granular leukocyte series.