The paper presents an assessment of the phagocytic ability of blood cells of Nile tilapia Oreochromis niloticus L. under the hormonal induction of stress with range of duration. It has been revealed that Staphylococcus aureus is phagocytosed by leukocytes, platelets, and erythrocytes of tilapia blood in direct smears. The phagocytic capacity of the cells has been quantified.