The immunostimulatory property of glucan isolates from three marine yeasts (Debaryomyces hansenii S8, Debaryomyces hansenii S169 and Candida tropicalis S186) and one Baker's yeast (Saccharomyces cerevisiae S36) as examined for potential application as immunostimulants in Penaeus monodon postlarvae against White Spot Syndrome Virus (WSSV) infection. Structural characterization of the glucan component in the isolates by proton nuclear magnetic resonance (NMR) indicated similar structures containing (1-3)-linked anhydroglucose repeat units (AGRUs) in the backbone with (1-6)-linked AGRUs in side chains that are (1-6)-linked to the backbone AGRUs. Glucan from C. tropicalis (S186) with the highest molecular weight and the lowest level of branching supported maximum survival (69%) followed by the other two marine yeast (S169 and S8) glucans of 27% and 23% respectively while glucan from Baker's yeast, S. cerevisiae S36 with the lowest molecular weight and the highest level of branching exhibited poor survival (4%) in P. monodon post challenge WSSV. The present study showed that the glucan isolate from marine yeast with a higher molecular weight and a lower degree of branching acts as better immunostimulants in P. monodon postlarvae than did the glucan isolate from S. cerevisiae.