This paper is the first of a series reporting the results of a study on haematology of Tilapia zilli (Gervais) in Egyptian waters. Erythrocyte count, total and differential leucocyte counts and their seasonal variations were studied. A complete description of the different types of leucocytes in the blood of T. zilli is given in detail. T. zilli is found to have medium sized red blood cells compared to other fishes. The analyses showed that it is one of the species having the highest leucocyte count. Polymorpho-nuclear leucocytes are not recorded in the blood smears of this fish.