We evaluated the effect of three different doses of Bacillus cereus as a probiotic added to rearing water on thenumber of bacteria, water quality, and disease resistant ability of Litopenaeus vannamei compared with those in a controlgroup. Shrimp were divided into four groups treated with different probiotic doses: group H (106 CFU/mL), groupM (105 CFU/mL), group L (104 CFU/mL), and the control group (0 CFU/mL). The intestinal tracts of the shrimp weresampled every 10 days to count total bacteria and Vibrio spp.. A water sample was collected at 8: 00 every 5 days. Ammonia-nitrogen and nitrite levels were measured by spectrophotometry. The shrimp were challenged with white spotsyndrome virus (WSSV) after 4 weeks, and dead shrimp were removed and recorded every day until the end of the experiment.Quantitative real-time polymerase chain reaction (PCR) analysis was performed to evaluate the expression ofthree immune-related genes after 0, 12, 24, 48, 96, and 192 h of exposure. The results showed that adding the probioticsignificantly reduced the number of bacteria in the L. vannamei intestinal tract compared with that in the control group.It also reduced the number of Vibrio spp. in culture water (P<0.05). However, no differences in ammonia-nitrogen ornitrite levels were detected, which contrasts with most previous studies. Cumulative shrimp mortality rates in group H(106 CFU/mL) and group M (105 CFU/mL) were 63.9% and 74.6%, respectively, after WSSV challenge, which weresignificantly lower than that in the control group with 100% mortality (P<0.05). Lipopolysaccharide, β-1, 3-glucanbindingprotein, and beta-1, 3-glucan-binding protein-lipoprotein mRNA levels were significantly upregulated 48 h afterthe WSSV infection, compared with those in the control group (P<0.05). Heat shock protein 70 mRNA expression wassignificantly upregulated 96 h after the WSSV infection compared with that in the control group but the difference betweenthe three experimental groups was not significant. We conclude that B. cereus as a probiotic added to rearing waterreduced the number of bacteria in the intestinal tract of L. vannamei and Vibrio spp. in culture water (P<0.05) anddid not increase ammonia-nitrogen or nitrite level. Adding B. cereus to the water improved the survival rate ofWSSV-infected L. vannamei. Moreover, B. cereus stimulated the expression of immune-related genes, which could improvedisease resistance.