Bioflocs are heterogeneous mixtures of microorganisms and organic particles that regulate the accumulation of ammonia and nitrite and serve as a supplemental food source for fish. A 56-day feeding experiment was conducted to investigate the effects of bioflocs on the growth performance, the activity of digestive enzymes, and non-specific immunity of (37.75±0.62) g juvenile Megalobrama amblycephala in zero-water exchange culture tanks. Glucose at different concentrations were used as the carbon source in the water. A series of C/N ratios including 8 (CN8), 12 (CN12), 16 (CN16) and 20 (CN20) were presented in biofloc-based tanks under zero-water exchange condition. Three biofloc treatments and one control treatment were applied to a triplicate. It was found that at the end of the experiment the survival rates of all the groups were >90% without significant differences between groups. The growth performance indicators such as the final weight, the weight gain and the specific growth rate were significantly higher (P<0.05) in the CN16 and CN20 groups than in the control group. Moreover, the biofloc treatments significantly enhanced activities of protease and amylase in the intestine of fish (P<0.05), but these treatments did not alter the body composition. We also observed marked differences in the activities of superoxide dismutase, alkaline phosphatase and lysozyme of the fish liver between the biofloc treatment groups and the control group. These results suggested that bioflocs in the CN16 and CN20 groups could improve the growth performance and feed utilization of cultured juvenile M. amblycephala, probably through the enhanced intestinal digestive enzyme activity and the subsequent feed digestion and utilization.