Marine secondary metabolites (MSM) from Hypnea musciformis positively affected immune fac- tors in post larvae of the shrimp, Penaeus monodon. Shrimp were raised in one of three salinity levels and infected with one of three doses of virulent Vibrio alginolyticus. When challenged with 105 of 106 V. alginolyticus cells/ml, survival was 100% for both control shrimp and shrimp fed a commercial shrimp feed treated with the MSM extract, regardless of salinity. But, when chal- lenged with 107 V. alginolyticus cells/ml, survival was 0 in control shrimp at all salinities, and 0, 34%, and 15% in shrimp fed treated feed and raised in 7, 17, and 27 ppt salinity, respectively.