The purpose of this study was to evaluate the potential of a host-associatedBacillus subtilis1-C-7 as a probiotic for Chinese perch (Siniperca chuatsi). Four test diets were formulated to contain graded levels ofB. subtilis1-C-7 at 0 (CY), 0.85 × 108(Y1), 0.95 × 109(Y2) and 0.91 × 1010(Y3) CFU/kg diet. The test fish with initial weight 30.0 ± 1.2 g were fed the 4 test diets with 3 replicates in an indoor water-flow aquaculture system with 12 net cages (40 fish/cage) for 10 wk. At the conclusion of the feeding trial, the probiotic effects ofB. subtilison Chinese perch were analyzed based on growth performance, serum biochemical indices, histologic morphology of liver and gut, gut microbiota and the resistance toAeromonas hydrophila. The results showed that the percentage of weight gain had no significant change in the Y1 and Y2 groups (P> 0.05) but decreased in the Y3 group compared to that in the CY group (P< 0.05). The fish in the Y3 group displayed the highest activity of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) among these 4 groups (P< 0.05). The fish in the CY group had the highest value of malondialdehyde in the liver (P< 0.05) and showed severe nuclear migration and vacuolization of hepatocytes. The morphology indicated that all test fish had poor intestinal health. However, the fish in the Y1 group had a relatively normal intestinal histologic structure. The mid gut microbial diversity analysis showed that dietaryB. subtilissupplementation increased the abundance of probiotics such as Tenericutes andBacteroides, whereas it reduced the abundance of pernicious bacteria such asProteobacteria,Actinobacteria,ThermophiliaandSpirochaetes. The challenge test showed that dietaryB. subtilissupplementation increased the resistance toA. hydrophilain Chinese perch. In conclusion, dietary supplementation of 0.85 × 108CFU/kgB. subtilis1-C-7 could improve the intestinal microbiota, intestinal health and disease resistance in Chinese perch, but more or excessive supplementation could reduce growth performance and have negative effects on health.