ABSTRACThe present investigation was conducted to study the effect of dietary supplementation of the probiotics Bacillus subtilis, Lactobucillus plantarum, a mixture of both bacterial isolates (B.subtilis and L. plantarum) and the yeast, Sacchromyces cerevisiae on the immune response of the Nile tilapia, Oreochromis niloticus. Two experimental diets were conducted. In the first experiment O. niloticus ( 24.55±0.03g) were fed for 60 days on four experimental diets supplemented with B. subtilis (10 7 CFU/g ) (D1), L. plantarum (10 7 CFU/g) (D2) a mixture of B. subtilis and L. plantarum (D3) and the yeast Sacchromyces cerevisiae (D4) and a control non-supplemented diet. All of the four experimental diets showed significantly increased phagocytic activity, acid phosphatase activity, lysozyme activity and total immunoglobulin activity in blood samples of the fish as compared with the control nonsupplemented diet. In the second experiment, O. niloticus (35.05±0.01g) were fed on a diet supplemented with three concentrations of L. plantarum (10 5 , 10 7 and 10 9 CFU/g) (D1, D2 and D3) respectively for 60 days. The probiotic treatment increased significantly the phagocytic activity for fish fed D2, while acid phosphatase activity was significantly increased for fish fed D3. Diets (D2 and D3) showed significantly the highest levels of lysozyme activity. No significant differences between the three diets for phenoloxidase activity, while total immunoglobulin showed significantly increased levels for fish fed D1 and D3 respectively. The results of the present study support the use of probiotics as immunostimulants in common fish diets.