Four diets from commercial sources were offered to 120 Nile tilapia (Oreochromis niloticus) divided into four equal groups. The first group was fed a control diet (treatment 1) without supplementation. The second, third, and fourth groups were fed diets supplemented with 3, 5, and 7 gm, respectively, of Bactocell® brand Pediococous acidilactici / kg feed. The number of Aeromonas hydrophila and Listeria monocytogenes isolated from internal organs of experimental fish (muscles and liver) every month for three months was significantly lower in all treated groups than in control. The lowest record was in the diet supplemented with 3g/kg Pediococous acidilactici bacteria. PCR was considered the most sensitive method for the detection of Listeria monocytogenes. The percent level of protection among the three treated groups after challenge infection using A. hydrophila (0.5 ml of culture suspension of pathogen containing 107 bacteria ml-1) was higher than control and the highest record was in the 7g/kg Pediococous acidilactici supplemented diet. The results of the present study support the use of Pediococous acidilactici bacteria at a rate (7g/kg standard fish diets) supplementation as immunostimulants and growth promoters in common fish diets.