We studied the effects of a combination of dietary beta-glucans (β-G) and mannan oligosaccharides (MOS) on Nile Tilapia Oreochromis niloticus. Three-hundred-sixty fingerlings (mean mass ± SD = 8.7 ± 0.4 g) were separated into three groups (G1, G2, and G3) of 120 fish; G1 (control group) was fed a basal diet, whereas G2 and G3 were fed prebiotic-supplemented diets at final levels of 1.5 and 3.0 g/kg feed, respectively. Each group was subdivided into two subgroups: subgroup A was fed for 60 d to evaluate growth performance, nutrient utilization, intestinal morphometry, and body composition; and subgroup B was fed for 30 d to evaluate immune status and disease resistance. The best growth and feed utilization were observed in G3. There was no significant difference in final body weight or weight gain between G2 and G1 after 30 d, whereas both variables were significantly higher in G2 than in G1 after 60 d. At the end of the feeding period, G2 had a better feed conversion ratio than G1. Villus height, number of goblet cells, and number of intraepithelial lymphocytes were greatest in G3, followed by G2 and then G1.Whole-body protein content and fat content were higher in G3 than in G2 and G1. Only G3 had significantly higher serum total protein, albumin, and globulin than G1. Serum killing percentage and phagocytic activity were significantly higher in G3 than in G1 and G2, whereas serum lysozyme activity was significantly higher in G3 and G2 than in G1. The nitric oxide assay indicated a significant effect in G3 compared with G1 after 30 d. Fish that were fed the prebiotic mixture had better relative percent survival than G1 fish after challenge with Yersinia ruckeri. Dietary supplementation with β-G and MOS in combination improves the performance of Nile Tilapia.