Bacterial diseases have been reported to be the major limiting factor in shrimp production. The use of natural products such as antimicrobials has been reported as a solution to the problem. The crude extract of a red seaweed Laurencia snyderiae obtained from the Persian Gulf was evaluated for shrimp growth performance and to determine in vivo efficacy of this seaweed in the prevention of shrimp Vibriosis. The ethanol extract from L. snyderiae (EELS) that was fed to the Artemia instar I for their enrichment was found to be non toxic to them. Shrimp Litopenaeus vannamei juveniles were fed with these enriched Artemia at 0 mg mL-1 (Control group), 200 mg mL-1, 400 mg mL-1 and 600 mg mL-1 for 30 days. The results obtained showed a significant increase (p < /em><0.05) in survival rate in treatment groups compared with that in the control group. Shrimps fed with enriched Artemia showed a significant improvement in growth parameters when compared to those in the control group. When these juvenile shrimps were exposed to Vibrio harveyi (after 30 days) they showed notably lower mortality than the control. These results indicate that EELS has a good potential in promoting growth and antibacterial activities against V. harveyi that is useful in shrimp aquaculture.