This study was carried out to evaluate the effects of Bacillus subtilis and molasses on water quality and growth of tilapia (Oreochromis niloticus) under zero-water exchange.Four experimental groups including treatments with molasses alone (C),Bacillus subtilis alone (Y), molasses and Bacillus subtilis (C+Y) and the negative blank were designed.During the experiment,water temperature ranged from 26 to 31℃ and pH ranged from 6.5 to 7.5.The mean dissolved oxygen was significantly higher in the blank than the 3 treatments (P＜0.05).The groups with utilization of molasses alone and together with B. subtilis had significantly higher total ammonia contents (P＜0.05).Combined utilization of molasses and B. subtilis showed the lowest level of nitrite,nitrate nitrogen and soluble reactive phosphorus.Overall,phytoplankton density in all groups was rather low (104-106 cells/L),but the three treatments showed higher phytoplankton density than the blank during the late period of the experiment.Survival rate,growth rate of tilapia and feed efficiency were higher in the 3 treatments than that in the blank,but the difference is not significant (P＞0.05).