The basal diet containing 24.5% fish meal as control,and experiment diet(YE group)with 2.5% yeast extract in basal diet,were fed to the white shrimp Litopenaeus vannamei respectively for 56 d,then the mRNA expression levels of Toll receptor,IMD and lysozyme in gills and Vibrio-resistance of the both shrimp were detected for assessing the effect of dietary yeast extract on sensitivity and balance of immune-related gene expression of shrimp.The results showed that:The shrimp fed the experiment diet displayed significantly increased mRNA expressions of Toll receptor and lysozyme in gills compared with the shrimp fed the control diet(P<0.05).There was no significant difference in IMD mRNA expression level in gills between YE group and the control(P>0.05).The relative mRNA expression levels of Toll receptor,IMD and lysozyme of the two groups changed significantly after Vibrio alginolyticus challenge,and the peak of the gene expressions appeared at 24,42 and 36 h respectively in both groups.The peaks of the gene expression of Toll receptor,IMD and lysozyme in YE group were 201.06%,481.46% and 276.77% of those in the control,and significantly higher than those in the control(P<0.05).The mRNA expression levels of Toll receptor and IMD in YE group significantly up-regulated at 12 and 24 h respectively while at 24 and 42 h respectively in the control after Vibrio challenge.There was no significant difference in 72 h cumulative mortality after V.alginolyticus infection between the two groups.It is therefore suggested that yeast extract added in diet significantly up-regulated the relative mRNA expression levels of Toll receptor and lysozyme in gills of L.vannamei before V.alginolyticus challenge,led to significant up-regulations of the relative mRNA expression levels of Toll receptor and IMD early and increased the peaks of the relative mRNA expression levels of Toll receptor,IMD and lysozyme in gills of the shrimp after V.alginolyticus challenge.This result implied that dietary yeast extract improved the sensitivity of the immune related genes expression of the shrimp to some extent.