The structure and topography of the Bursa of Fabricius (BF) differs from other central immune organs. Structural components, providing BF function are located in mucosa, which is constantly influenced by antigen stimulation. Therefore the reports, which have appeared recently, indicate that BF may function as a peripheral immune organ. The latter is known to be situated in the alimentary canal mucosa (caecal tonsils, Peyer's patches) and possess peculiar epithelium. The aim of our study was to investigate the structure and cell composition of BF epithelium in chicken, turkey, quails, ducks and pigeons using light and electronic microscopy. The studies have shown that epithelium overlying BF folds is pseudostratified columnar, but between them and in their base it might be cuboidal or columnar. Pseudostratified epithelium consists of columnar cells with a few microvilli on their apical surface, goblet cells, infrequent endocrine cells and macrophages. BF epithelium is spongious due to its infiltration by lymphoid cells. The large quantity of the latter is found in subepithelial connective tissue. Infiltration by lymphoid cells is considered to be typical for the alimentary tract mucosa, where the aggregated lymphoid nodules are situated. Thus, our study supports the thoughts that BF may have function of central and peripheral immune organ.