An experiment of 90th days duration was conducted to test the efficacy of the fucoidan derived from brown seaweed Padina tetrastromatica as an immunostimulant in Giant freshwater prawn M.rosenbergii. The prawns were fed with the diets containing different concentration of fucoidan viz., 1%, 2% and 3%. The proPhenol Oxidase (proPO) activity and the NBT activity was found to be elevated in the experimented group fed with fucoidan derived from P.tetrastromatica on 30th and 45th day at 3% concentration respectively. The results of the study reveal that supplementation of dietary fucoidan @ 3% could improve selected immune parameters of M.rosenbergii