The synergistic effects of probiotic (Bacillus pumilus) and/or exogenous enzyme (protease) on growth, immunity, serum parameters, gene expression and gut bacteria of Nile tilapia (3.62 ± 0.06 g) were investigated. Plant protein-based diets were formulated as the control diet, probiotic (pro, 1.85 × 105 B. pumilus CFU kg−1), protease enzyme (enzy, 0.5 g protease kg −1), and their mixture of 1.85 × 105 B. pumilus CFU kg−1 + 0.5 g protease kg −1 (pro-enzy), respectively. After 84 days, the results revealed that fish fed pro-enzy diet resulted in better (P < 0.05) growth performance and feed utilization, including highest goblet cells, thickness of musclaris, mucosal folds and enterocytes. In addition, the highest values of hematocrit (Htc), hemoglobin (Hb), red blood cells (RBCs), white blood cells (WBCs), immunoglobulin M (IgM), lysozyme and phagocytic and serum parameters, including albumin, globulin and total …