A histological investigation of the possible structure and functions of the spleen of the Niletilapia Oreochromis niloticus was cond ucted by light (LM}, and transmission electron microscopy (TEM). The spleen is divisible into red and white pulp and a stroma consisting of a reticular network, a collagenous capsule, and trabeculae containing smooth muscle bundles. The parenchyma of the organ is predominantl y made up of the red pulp, a system of splenic cord s and sinuses. White pulp areas appear to be devoid of lymphoid follicles and consist mainly of periarteriolar lymphatic sheaths (PALS). Fil tering of particulate matter from the blood occurs in the red pul p by phagocytes of the pulp cord s and ellipsoid s (periarterial macrophage sheaths). The ellipsoid s are pale staining cu.Ifs of macrophages and reticular cells in a framework qf reticular.fibres sur rounding the arterial capillaries. The melano-macrophage centres (MMCs) are found throughout the parenchyma of spleen and show a close association with the vascular system. i.e. splenic ellipsoid s,. sinusoid s of red pulp and blood sinuses. They exhibit d istinct degree of development from small groups of actively phagocytic macrophages to large. totally or partiall y encapsulated centres, where effete phagocytic cells are filled by lipqfuscin and melanin. The ultrastructural data presented here suggest various specific physiological roles for the Nile tilapia Oreochromis niloticus spleen, incltLd ing hematopoiesl.s , phagocytosis. tissue breakdown and erythrocyte catabolism. Only sparse lymphopoiesis and plasmapoiesis were recognized