Seasonal changes in the Thymus gland of Tilapia Nilotica Fish *Attia, H.F ; I.M.A El-Zoghby; ; Mona N. A. Hussein and **H. H Bakry *Dept. Histology &Cytology and ** Dept of Forensic medicine. Fac. Vet. Med. Benha University This study was carried out on 100 thymus gland of apparent healthy Tilapia Nilotica fish. The thymus was situated on the superior edge of the gill cover close to the opercular cavity; the thymus was surrounded by connective tissue capsule which consisted of collagen fibers and reticular fibers. The thymocytes (T- lymphocytes) form the main type of cells present in thymus embedded within network of epithelial reticular cells. The thymus was characterized by presence of hassall’s body-like structures, myoid cells, mucin secreting cells and melanomacrophage cells. The thymus showed great involution in winter in form of decrease number of lymphocytes, proliferation of connective tissue and adipose tissue. Thymus showed regeneration of tissue during spring. Sharp increase in lymphocytes number was observed during summer as the lymphocytic foci became denser and increased in size . In autumn the parenchyma was divided into cortico-medullary like zones with increase in connective tissue proliferation especially in the medulla. The lymphocytic foci become less dense and decreased in size.