A combination of probiotics and prebiotics as synbiotics allows assessing their synergistic effects. This study evaluated the effect of synbiotic (Lactobacillus acidophilus and fructooligosaccharides+mannan oligosaccharides, FMOS) on growth, hematological parameters, plasma hormonal, and genes expression in Oreochromis niloticus. A total of 600 O. niloticus of an average initial weight ranged from (4.96 to 5.96 g) was divided into four experimental groups for 84 days. Four basal diets Diet 1 (control), Diet 2, Diet 3 and Diet 4 were formulated to contain four levels of L. acidophilus (0.00, 0.42×107, 0.84×107 and 1.35×107CFU g-1) respectively, each level supplemented with 1% FMOS except of the control diet. O. niloticus fed diet supplemented with synbiotic showed significant (P