Abstract: This paper reports the completely dif ferent results of positive selective breeding and negative selective breeding of blunt snout bream , Megalobrama amblycephala in 14 years. The positive selective breeding is by mass selection, the total selective intensity from fry to brooder was about 0. 04%. Large and healthy individuals were selected, the average daily body weight growth rate of selective strain F5 was 29. 1% higher than the control group, e. g. , 5. 8% selective response per generation. The negative selective breeding is by full..sib matting, which off springs are kept randomly. The average daily body weight growth rate of adult f ish of the 3th generation of inbreeding group was 16. 6% lower than the contro l group, e. g. , - 5. 5% selective response per generation. In the 4th generat ion of inbreeding group, the deformity rate of fingerlings was 2. 8%, survival rate overwintering was 28. 4% lower than the control group, and 48. 3% lower than the positive selective group. This study proved that the aquaculture performance can be improved signif icantly by scientif ic selection, but depressed rapidly by inbreeding, which should be prevented strictly.