Monogenean parasites pose a considerable threat to the aquaculture of yellowtail kingfish, Seriola lalandi. Current chemotherapeutic treatments are expensive, labor intensive and can cause mortality if incorrectly administered. Many medicinal herbs possess anti-parasitic properties and therefore have potential as an alternative or supplement to current treatment methods. The anthelminthic effects of garlic, rosemary, and commercially manufactured herbal products were trialed both in vitro and in vivo against two species of monogeneans, Neobenedenia girellae and Zeuxapta seriolae, to assess their suitability for use in yellowtail kingfish aquaculture. The survival of adult N. girellae and Z. seriolae and larval N. girellae (i.e. oncomiracidia) were assessed in vitro, by exposure to seawater solutions containing the herbal products. All herbal-seawater solutions significantly reduced the survival of mature N. girellae and Z. seriolae and N. girellae oncomiracidia relative to the controls. Two in vivo trials were then conducted to examine the preventative and curative treatment effects of rosemary and garlic against Z. seriolae infections, by supplementing these products into their feed. The first trial fed supplemented diets for 30 days prior to exposure with Z. seriolae oncomiracidia. The second trial examined the potential curative effects of these extracts by feeding supplemented diets to yellowtail kingfish already infected with Z. seriolae for 10 and 20 days. The effect of these curative treatment diets on recruitment of juvenile Z. seriolae was also assessed. Supplementation of rosemary and garlic products into feed 30 days prior to experimental exposure significantly reduced Z. seriolae infection success, while supplementation of garlic powder, rosemary extract, and rosemary oil onto pellets significantly reduced Z. seriolae abundance on yellowtail kingfish after 20 days of feeding. Recruitment of juvenile Z. seriolae was also significantly reduced.