Abstract: To evaluate the effects of feeding and fasting on swimming performance and hypoxia tolerance of fish, we tested the routine metabolic rate (RMR), active metabolic rate (AMR), metabolic scope (MS), critical swimming speed (U_crit), critical oxygen pressure (P_crit) and the point of loss of equilibrium (LOE) of largemouth bass (Micropterus salmoides) at 25℃. The fish were divided into 3 groups which include control group (fasting for 2d), feeding group (3h after feeding) and fasting group (fasting for 16d). The results showed that the RMR increased, but the MS and U_crit decreased after feeding (P<0.05). There was no change in AMR between feeding group and control group (P>0.05). There was no change in RMR, AMR and MS between fasting group and control group (P>0.05). The U_crit of fish was decreased after fasting (P<0.05). The P_crit was higher in feeding group than that in control group (P<0.05). When the dissolved oxygen level was higher than the P_crit, the slope of the metabolic rate (MR) and dissolved oxygen (DO) in feeding group was higher than that in control group (P<0.05). There was no difference in LOE between feeding group and control group (P<0.05); There was no difference in P_crit and LOE between fasting group and control group (P<0.05). However, when the dissolved oxygen level was below the P_crit, the slope of the MR and DO in fasting group was lower than that in control group (P<0.05). The results showed that the weakening of swimming performance of largemouth bass in feeding group was caused by the limitation of its cardiobranchial system on its aerobic metabolism ability. After fasting, the weakening of swimming performance of largemouth bass may be related to the decline of anaerobic metabolism ability; Feeding weakens the hypoxic tolerance of largemouth bass, while fasting enhances its hypoxic tolerance. However, the hypoxic tolerance of largemouth bass is conservative.