Aims: The objective of the work was to determine whether known strains of nonpathogenic vibrios can act as probiotics for the control of Vibrio infections in the Pacific white shrimp, Litopenaeus vannamei. Methods and Results: Of the ten species tested, only Vibrio alginolyticus (NCIMB 1339) and Vibrio gazogenes (NCIMB 2250) showed antagonistic activity towards a panel of shrimp pathogenic vibrios. In the case of V. alginolyticus, this activity depended on the presence of live bacteria while in V. gazogenes both live and dead bacteria showed anti-Vibrio activity. Injection of shrimp with either V. alginolyticus or V. gazogenes at 3 x 107 or 3 x 105 total bacteria per shrimp resulted in mortality with higher levels in the case of V. alginolyticus (100% mortality 18 h postinjection of 3 x 107 bacteria). Juvenile shrimp were fed commercial diets top-coated with either chitin (an immune stimulant) or chitin + V. gazogenes. Both chitin and V. gazogenes caused a significant decline in the number of Vibrio-like bacteria in the fore and hind gut, and changes were also seen in the hepatosomatic index (a measure of digestive health) and the total number of blood cells in circulation. Analysis of mid/hindgut and faecal samples obtained using terminal restriction fragment length polymorphism showed that the gut microbiota of shrimp has limited bacterial diversity and that after 8 weeks exposure to the experimental diets there were significant changes in the microbial flora of the GI tract of shrimp as a result of the presence of V. gazogenes. Conclusions: Of the vibrios tested, V. gazogenes has potential as a probiotic for the control of bacterial diseases in shrimp. Significance and Impact of the Study: Overall, this study shows the promise of V. gazogenes together with chitin to improve the health and welfare of shrimp under aquaculture conditions.