A 45-day feeding trial was conducted to evaluate the effects of alginic acid (Ergosan®) supplementation in Pacific whiteleg shrimp, Penaeus vannamei juvenile. Shrimps (0.6 g ABW) were fed diets supplemented with different levels of alginic acid as immunostimulant. Experimental diets were formulated to contain 0.2, 0.4 and 0.8% and a control. The feeding trial was conducted in 50-L capacity rectangular plastic container stocked with 20 shrimps each in triplicates. Growth, survival, resistance to Vibrio parahaemolyticus, total haemocyte count (THC), respiratory burst activity, phenoloxidase activity, and clearance efficiency were evaluated. Results showed that growth and survival were not affected by supplementation of immunostimulant. On the other hand, bacterial challenge showed 100% mortality in 0.8% alginic acid fed group though not significant with the control. Total haemocyte count, respiratory burst and phenoloxidase activity were significantly enhanced in the group supplemented with 0.4% and 0.2% of alginic acid. Immune responses of the group fed with the highest concentration (0.8%) were significantly suppressed. The same trend was obtained for the clearance efficiency. The present results demonstrated that using alginic acid less than or equal to 0.4% activates immune responses and resistance against vibriosis, otherwise overstimulation of the immune indices could cause immunosuppression.