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This study tested the onion powder (OP) supplementation in the diet of the olive flounder on the growth, body composition, and lysozyme activity. Thirty-five fish averaging 5.1 g were randomly stocked into 18 individual 180-L flow-through tanks. A commercially available OP was used as a dietary additive. Six experimental diets were prepared to contain OP at the concentrations of 0, 0.5, 1, 2, 3, and 5% for diets OP-0, OP-0.5, OP-1, OP-2, OP-3, and OP-5, respectively. After the 8-wk feeding trial, 20 fish from each tank were infected with Edwardsiella tarda and mortality was monitored for the following 96 h. No distinctive improvement in survival, weight gain, or feed efficiency of fish was observed at the end of the 8-wk feeding trial. Lysozyme activity in fish fed the OP-0.5 diet was higher than that of fish fed the OP-0, OP-1, OP-2, OP-3, and OP-5 diets. The cumulative mortality of fish fed the OP-0.5, OP-1, OP-2, OP-3, and OP-5 diets was lower than that of fish fed the OP-0 diet at 72 h after E. tarda infection. Dietary inclusion of 0.5% OP was effective at improving lysozyme activity of fish, and OP seemed to be an effective immunostimulant to lower mortality upon E. tarda infection.