A 12-week (September to December 2009) feeding trial was conducted to evaluate theimmunostimulatory effects of different substances administered orally through the diet in the brown-marbled grouper, Epinephelus fuscoguttatus. Five experimental diets containing either onion, ginger, β–glucan, or vitamin C and a control diet (without immunostimulants) were fed to the fish weighing about 44 g for 12 weeks. Onion-fed fish showed significantly increased weight gain, hematocrit, and total Ig compared to the control group; however, leukocyte differential count and ROS production were unaffected. Ginger-fed fish likewise significantly increased total Ig, ROS production and lysozyme activity. However, it did not affect growth and hematocrit value. β-glucan significantly increased growth and total Ig but had no effect on the other parameters. Vitamin C significantly increased hematocrit, total Ig and ROS production but did not increase growth. Upon challenge with a bacterial pathogen Vibrio harveyi, mortality was significantly reduced in the onion, ginger and vitamin C-fed fish but not in the β–glucan-fed fish. This study demonstrated that onion and ginger could positively affect the innate immune responses and protect grouper against Vibrio harveyi infection.