In this study, histological changes occurring postnatally in chicken bursa Fabricii were investigated by tight microscopic techniques.For this purpose, bursa Fabricii samples taken trom 60 domestic hybrid line, GXSX chic kens were used as a material. The tissue samples immersed in parattine blocks by means of routine histological procedures, following fixation in alcoholic-formaline and Hefty fixatives. Tissue sections taken at 6 pm in thickness were stained with Grossman's trichrorne, Pappenheim's panoptic and methyl green-pyronin staining methods. In the first three weeks of the posthatching period, lymphoid follicles growed gradually and occupied lamina propria ot the organ. The lymphoid follicles extended and gained lamp glass shape because of the compression. Any definite histological changes related lo the involution of the organ were not observed in both interfollicular epithelium (IFE) and follicle associated epithelium (FAE) by the 12 th week of the posthatching period. The early histological evidence ot involution such as intraepithelial and intralobular cysts were first observed in 12 th week ot posthatching period. As the age progressed, the cysts enlarged and int raf olive ular cysts occupied medullae ot the follicles. In the 14 th week of post-hatching period, the intralollicular cysts opened and cyst content discharged into the centrally located lumen through degenerated FAE region of the surface epithelium. The lymphoid follicles have lost their typical histological organizations and the organ had a multitubular appearance with a number of tubuli opening into the central lumen. Lymphocytic infiltration areas which were first observed in the posthatching 14 th week have expanded in 23 rd week. At the same period, a number ot pyroninophylic plasma cells were also observed in the lamina propria which were highly vascularized and had increased amounts ot fibrous and cellular connective tissue elements. These histological findings have revealed that bursa ot Fabricius, which has been regarded as a B-lymphocyte producing centrally lymphoid organ, may serve as a secondary lympoid organ at the late and residual periods of involution.