This study aimed to evaluate gut colonization by probiotic bacteria, and their influence on the bacterial microflora, hematological profile, growth rate and proximate composition, and water quality parameters in the cultivation of Nile tilapia juveniles of the Genetically Improved Farmed Tilapia (GIFT) strain. 1800 fry were randomly distributed into four experimental groups (T1= basal diet (BD) without probiotic addition; T2 = BD + 0.5% Bacillus cereus var. toyoi (BC); T3 = BD + 0.5% B. subtilis C-3102 (BS); T4 = BD + 0.25% BC + 0.25% BS) with five replicates. Cages (0.175 m³ capacity) containing 90 tilapia fry (0.34 ± 0.06 g) were used as culture units and individually installed in 8.4 m³ ponds. Except for hemoglobin, hematocrit, glucose, and neutrophil levels, no differences in hematological profile were observed among the groups after 127 days of culture (P > 0.05). The probiotic bacteria either added individually or in combination, successfully colonized the intestines of the fish, without negatively influencing the intestinal bacterial microflora, growth rate, proximate composition, or water quality parameters (P > 0.05). Thus, dietary probiotic supplementation of juvenile Nile tilapia alters the hematological profile, especially hemoglobin, hematocrit, glucose, and neutrophil levels.