Previous studies in several fish species have shown efficacy of a low-dose garlic compound, allicin, against several pathogens. In this study, young rainbow trout (Oncorhynchus mykiss) were fed diets containing 0%, 0.5%, 1.0% and 2.0% allicin and then challenged with a modified LD50 dose of Aeromonas salmonicida and monitored for 28 days. There were significant increases in survivability in the 0.5% and 1.0% groups and a significant reduction in survival at the 2.0% dose. A target animal safety was performed using the target dose of 0.5% allicin at 0x, 1x, 3x, and 5x the target dose for 3x the duration (6 weeks) of the original study. There were no statistically significant differences in erythrocyte parameters or average leukocyte counts; however, there was a significant decrease in the level of circulating monocytes in the high dose (2.5%) group. This correlated to an increased level of pigment-containing macrophage centers within the renal tissue as allicin dosing increased, denoting a potential inflammatory effect. On serum chemistry, glutamate dehydrogenase (GDH) was elevated, most likely as a result of increased cellular metabolism and increased hepatocyte turnover due to allicin dosing. Initial studies indicate that feeding 0.5% allicin improves survivability when challenged with A. salmonicida and appears safe; however, higher levels may cause negative effects on health.