Phagocytosis by macrophages is one of non-specific immune mechanisms against pathogenic bacteria. Fishes treated with immunostimulants usually show enhanced phagocytosis [17,49,50]. In the study, phagocytosis was also enhanced with increasing mistletoe supplementation in the diet. However, phagocytes from 10 mg treated fish failed to elicit higher phagocytic activity than those from mistletoe non-fed control fish on days 20, 40, and 80 of feeding, suggesting that 10 mg mistletoe is insufficient to induce phagocytic activity throughout the feeding periods in this study. Conversely, on the post-challenge period, the phagocytic activity was significantly uo-regulated compared to the control fish. The pattern of the result is likely to correlate to that of ROS production except 80 days of feeding. Some researchrs have reported that ROS production by the fish fed with Achyranthes incorporated diets was derived from Achyranthes-stimulated phagocytes [51]. In this study, the level of respiratory burst and phagocytic activity in the 200 mg mistletoe-treated group, was not significantly higher than that in 50 mg mistletoe-treated group indicating that a mistletoe 50mg kg dry diet is adequate for stimulating maximum phagocytic activity.