The fermentable fiber, Vitacel R200 is industrial compound cellulose exerting many beneficial health effects at the hosts. In this study, rainbow trout (Oncorhynchus mykiss) fingerling (120 ± 4.5 grams) were fed dietary 1% (10 g/kg feed) Vitacel R200 for 7 weeks, then challenged with Aeromonas hydrophila. Two weeks after the challenge, hemato-biochemical and immune parameters in the challenged fish were studied. The lysozyme and complement activity, as well as, white blood cells (WBCs), hemoglobin (Hb%), percentage of neutrophil and eosinophil in the fish fed Vitacel were significantly increased compared with the control group. Furthermore, the serum total protein, albumin, Na+, K+, P, and Ca++ concentrations in the fish fed diet containing 1% Vitacel were significantly increased, while alanine aminotransferase (ALT), alkaline phosphatase (ALP), and aspartate aminotransferase (AST), as well as, cortisol, cholesterol, and glucose concentration were significantly decreased in comparison to the control group. These results concluded that dietary Vitacel could improve the hemato-biochemical and immune parameters, as well as, the stress response in O. mykiss infected with A. hydrophila.