In this study, the diet supplemented with 0.2% of GAS1-β-glucans was the most effective to protect rainbow trout juveniles against a bacterial infection with A. salmonicida achromogenes. After short-term β-glucan feeding (D15), the immune system of rainbow trout juveniles differentially responded to the two β-glucan doses tested depending on the type of β-glucans. The gene discussed is GAS1; the disease is bacterial infectious disease.