This study examined the effects of feeding of two dietary probiotics on growth performance and feed utilization, intestinal colonization and related health criteria, biochemical parameters, hematological indices, and immune parameters and protection against Yersinia ruckeri in Oncorhynchus mykiss during a period of 56 days. The study included 3 groups:1(a control group of fish, 2) a group of fish fed with a basal commercial diet supplemented with 2 × 10⁶ CFU g^-1 Pediococcus acidilactici (PA-group and 3) a group of fish fed with a basal commercial diet supplemented with  2.5 × 10⁸ CFU g^–1 Enterococcus faecium strain IR5 (EF-group). Each group was run in triplicate. The PA-group and EF-group showed significant improvement with respect to WG, SGR, FCR, PER, lactic acid bacteria (LAB) (%) in intestinal, intestinal colonization (log CFU g–¹), survival (%), RBCs, WBCs, hematocrit percentage (Hct %), and respiratory burst activity (RBA) levels, each of which was significantly higher than that in the control group (p<0.05). The WG and PER in the EF-group were significantly higher than that in the PA and control groups (p<0.05). Levels of immune-system response across a variety of measures, some of the measured hematological indices, LAB (%) in intestines, and survival rate (%) were higher in the PA group than that in the EF group on 56th days of feeding. Mortality in the PA group (20%) and EF group (33.3%) were significantly lower compared with that in the positive control (73.3%) during the 10 days following exposure to Y. ruckeri (p<0.05).