Garlic, Allium sativum, has had an important dietary and medicinal role for centuries. In this study the antibacterial effect of garlic extracts were evaluated against Photobacterium damselae subsp. piscicida, Aeromonas hydrophila, Edwardsiella tarda, Vibrio alginolyticus and V. harveyi by the agar diffusion assay. The aqueous garlic extract were found to be able to inhibit growth of all tested bacteria in a dose-dependent manner with maximum of 100% at defined concentration. The acetonitril garlic extract, aqueous garlic extracts with and without garlic were prepared the test diets and were fed to cobia for 9 days. Mortality for the cobia receiving the diets supplemented with WE-X, a treatment of the aqueous garlic extract without garlic, was lowest and significantly different (p<0.01) from that of the control.