The present research evaluated the value of adding 3, 5, and 7g of a probiotic mixture of Pediococcus acidilactici, Lactobacillus acidophilus, and Bacillus subtilis/kg fish feed on the physiological status and fish health of the Nile tilapia (Oreochromis niloticus). A total of 120 Nile tilapia fish were divided into four equal groups fed on pellets containing different ratios of the probiotic mixture for three months. After the experimental period, the whole-body compositions (fats, protein, and ash content) of tilapia were affected significantly in all groups (P≤ 0.05). Theimmunological parameters, nitroblue tetrazolium (NBT) and lysozyme activities significantly increased in all treated groups. The results showed significant hematological parameters [Hematocrit (PCV), hemoglobin (Hb), erythrocyte counts (RBC), Mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC)] and significant biochemical blood parameters [glucose, albumin, protein, globulin, aspartate aminotransferase (AST) and alanine aminotransferase (ALT)] measured in serum after the experimental period. The number of Vibrio sp. isolated from the internal organs of experimental fish (muscles and liver) every month for three months was lower significantly in all treated groups than in the control. The relative level of protection (RLP) among the three treated groups after infection-challenge withVibrio harveyi (0.1ml of culture suspension of pathogen containing 10⁸ bacteria ml^-1) was higher than in the control, and the highest record was in the fourth group.The present research results support the use ofa mix of Pediococcus acidilactici, Lactobacillus acidophilus, and Bacillus subtilisat a rateof7g/ kg standard fish dietssupplementation as immunostimulants in common fish diets.