farmed fish. Although there are pharmacological treatments against these parasites, they are not equally effective for all stages of their life cycle and could be harmful to fish and the environment, hence the interest in studying the therapeutic potential of plant-derived products. This study evaluated the anthelmintic effect of diallyl sulfide (DAS), a secondary metabolite of garlic, on immersion baths, through in vitro and in vivo trials, on the life stages of Zeuxapta seriolae. In vitro, we found that eggs exposed acutely or continuously at 0.01 mg DAS/mL exhibited reduced hatching success, the oncomiracidial lifespan was significantly reduced when exposed to 0.01 mg DAS/mL. In vivo, mean parasite intensity decreased significantly with 0.01 mg DAS/mL; higher concentrations of DAS had adverse effects on the fish, impeding treatment completion. These results add to the mounting evidence of the utility of garlic extract, or other isolated compounds from garlic like DAS to control Z. seriolae affecting California yellowtail and other commercially important species.