Two experiments were conducted effects of two dietary symbiotic; Bacillus subtilis with mannan oligosaccharides and Enterococcus faecium with fructo-oligosaccharides were evaluated on growth performances, hematological parameters and immune responses in Japanese eel, Anguilla japonica and Nile Tilapia Oreochromis niloticus for 8 weeks. Exp.1: Six experimental diets consist: a basal diet (CON), an antibiotic diet containing CON with 4 g kg-1 of oxytetracycline (OTC), and 4 synbiotic diets; Bacillus subtilis at 107 or 108 CFU g-1 plus 4 g kg-1 mannan oligosaccharides (BS7 and BS8 ) and Enterococcus faecium at 106 and 107 CFU g-1 plus 4 g kg-1 fructo-oligosaccharides (EF6 and EF7). Eighteen tanks of fish averaging of 6.00 ± 0.07g (mean ± SD) were randomly assigned into six experimental diets in 3 replicates. Weight gain (WG) and specific growth rate (SGR) of fish fed BS7 and BS8 diets were significantly higher than those of fish fed CON and OTC diets (P<0.05), however, there were no significant differences among those of fish fed BS7, BS8, EF6, and EF7 diets. WG and SGR of those of fish EF6 and EF7 were significantly higher than those of fish fed CON diets (P> 0.05). Feed efficiency of fish fed BS7, BS8 and EF6 diets were significantly higher than those of fish fed CON and OTC diets, however, there were no significant difference among fish fed BS7, BS8, EF6, and EF7; among those of fish fed EF7 and CON; and among those of fish fed OTC and CON (P> 0.05). Protein efficiency ratio of fish fed BS7, BS8, EF6, and EF7 diets were significantly higher than those of fish fed CON and OTC diets (P < 0.05). Average myeloperoxidase activity of fish fed BS7 diet were significantly higher than those of fish fed CON, OTC, BS7, EF6, and EF7 diets (P < 0.05). After the challenge test by Vibrio anguillarum, average cumulative survival rates of fish fed BS8 were significantly higher than those of fish fed CON diet, however, there were no significant differences among those of fish fed OTC, BS7, BS8, EF6, and EF7 diets (P > 0.05). Exp.2: Six experimental diets include a basal diet (CON), a positive control diet (OTC) with antibiotics (0.4% oxytetracycline of diet) and 4 synbiotic diets consisting the basal diet with B. subtilis (1×107 or 1×108 CFU g-1) with 0.4% of mannan oligosaccharide (BS7M or BS8M) and the basal diet with E. faecium (1×106 or 1×10 7 CFU g-1) with 0.4% of fructo-oligosaccharide (EF6F or EF7F). Eighteen tanks of fish averaging 4.8 ± 0.05 g (mean ± SD) were randomly assigned into six experimental diets with three replicates, and fish fed the experimental diets for eight weeks. Weight gain of fish fed OTC, BS8M, EF6F and EF7F diet were significantly higher than those of fish fed CON diet (P<0.05). Weight gain of fish fed EF7F diet was significantly higher than those of fish fed BS7M diet although there were no significant differences among fish fed OTC, BS8M, EF6F and EF7F diet. Specific growth rate of fish fed OTC, BS7M, BS8M, EF6F and EF7F diet were significantly higher than those of fish fed CON diet (P<0.05). Specific growth rate of fish fed EF7F diet was also significantly higher than those of fish fed BS7M diet although there were no significantly differences among fish fed OTC, BS8M, EF6F and EF7 F diet. These results suggest that four synbiotics; BS7, BS8 (Bacillus subtilis at 107 or 108 CFU g-1 plus 4 g kg-1 mannan oligosaccharides) EF6, EF7 (Enterococcus faecium at 106 and 107 CFU g-1 plus 4 g kg-1 fructo- oligosaccharides) could be the potential feed additive and antibiotics replacer in Japanese eel and Nile tilapia feeding and farming.