Abstract: The aim of this study were, to determine the anaerobic to aerobic metabolism ratio under the maximum sus-tainable swimming speed (critical swimming speed, Ucrit) of selected fish species, to examine at what swimming speed fish start to recruit its anaerobic metabolism, and hence to investigate the relationship among locomotion performance, energy supply characteristics and ecological habits of selected experimental animal. Juvenile darkbarbel catfish (Pelteobagrus vachelli Richardson) w: (4.34±0.13) g, an economic fish species distribute widely in the Yangtze River and Pearl River, were chosen as the experimental animal. The critical swimming speed (Ucrit) of juvenile Darkbarbel Catfish was determined at (25 ± 1)℃. Based on the Ucrit data, we set 7 experimental groups: 2 control groups (resting control group and high-speed exhaustive control group) and 5 speed-manipulating groups, which were set following the gradient of mean Ucrit (20, 40, 60, 80 and 100% Ucrit). Fish of speed-manipulating groups were forced to swim for 20 minutes at its setting speed, during this period, the oxygen consumption rate was measured and the activity metabolic rate was calculated. The lactate, glycogen and glucose levels of muscle, blood and liver of fish in experimental treatment groups were determined immediately after forced swimming. The biochemical parameters and resting metabolic rate (only for resting control group) were also measured in two control groups. The absolutely critical swimming speed of juvenile darkbarbel catfish in this study was (48.28±1.02) cm/s and the relative critical swimming speed was (6.78±0.16) BL/s. The oxygen consumption rate raised significantly with the increasing of the swimming speed (P 0.05). The mus-cle and blood lactate levels of fish underwent 100% Ucrit treatment were (7.25±0.70) μmol/g and (9.25±2.66) mmol/L while the lactate levles of muscle and blood samples were (5.31±0.43) μmol/g and (3.44±0.25) mmol/L, respectively. The lactate levels of both muscle and blood samples of 100% Ucrit treatment group were significantly higher than those of 80% Ucrit treatment group (P 0.05) while the lactate level of liver showed no significant change (P 0.05); As swimming speed increased, glycogen content of all three tissues showed a downward trend (P 0.05), of which liver glycogen content of 100% Ucrit group was significantly lower than that of 40% Ucrit level (P 0.05) while glycogen content of muscle showed no significant difference among all experimental treatment groups (P 0.05). Glucose levels remained relatively stable. Calculated anaerobic metabolism to aerobic metabolism power ratio was 11.0% when juve-nile Darkbarbel Catfish reached the critical swimming speed, indicating that the main power came from aerobic me-tabolism path; anaerobic metabolism started its function when fish reached around 80% of its Ucrit, the start-up time was late comparing with other fish. It showed that their swimming activity relied less on anaerobic metabolism. This study indicates that juvenile Darkbarbel Catfish has strong aerobic capacity and the characteristics of energy metabolism in this fish species may be related to its higher survival fitness.