Abstract: 3,3',4,4',5-pentachlorobiphenyl (PCB 126), a kind of typical persistent organic pollutant, is one of the poly-chlorinated biphenyls (PCBs) congeners with high bioaccumulation and strong toxicity. It has been considered to have detrimental effects on the health of the human and animals as well as on ecosystems for being difficult to be decomposed or be transformed in both environment and organisms. In order to test the lethal effect and metabolic stress of dietary PCB 126 on the southern catfish, Silurus meridionalis Chen, from October to December, 2007, 72 one-year-old juvenile south-era catfish acclimated with control diet for 2 weeks prior to study initiation were randomly divided into 6 experimental groups (12 fish in each group) for different dietary PCB 126 treatments. Six iso-nitrogenous (45% crude protein), iso-li-pidic (12% crude lipid) and iso-carbohydrate (15% crude carbohydrate) experimental diets with different levels of PCB 126 were formulated, the dietary concentrations of PC B 126 were 0(as control), 50, l O0,200,400 and 800 μg/kg, re-spectively. In each group, the southern catfish were individually fed with each experimental diet in an indoor rearing sys-tem at 3% BW per day ration level for 8 weeks at (27.5±0.2)℃. The median lethal time (LT50), the total amounts of PCB 126 intake during the experiment, hepatosomatic index (HSI), the resting metabolic rate (RMR) as well as the rate of oxygen consumption and activity of cytochrome c oxidase (CCO) of the liver mitochondria in southern catfish were deter-mined and calculated. The results showed that no mortality of the experimental fish occurred during the experiment in groups fed with the diets in the PCB 126 concentrations of 0, 50 and 100 μg/kg, and the total amounts of PCB 126 intake were 0, 30.56 and 66.66μg/kg body weight at the termination of the experiment, respectively, whereas, the mortality was observed in fish fed with the diets containing PCB 126 of 200, 400 and 800 μg/kg, and the LT50 was 34d, 16d and 11d, respectively, which were negatively correlated to dietary levels of PCB 126. The accumulative amounts of PCB 126 intake at the median lethal time in these three groups were 90.18, 92.05 and 94.11μg/kg body weight, respectively, and there was no significant difference among them, but all of them were significantly higher than those in the other three groups (p<0.05). HSI and RMR increased with the increasing dietary level of PCB 126. The rate of oxygen consumptionand activity of CCO of the liver mitochondria increased with the increasing dietary level of PCB 126 in the groups without mortalities, while decreased with the further increasing dietary level of PCB 126 in the groups with mortalities. The present study suggested that the accumulative lethal critical amounts of PCB 126 intake for the southern catfish were approximately 92 μg/kg body weight and the lethal time was concentration-dependent. The stress effects of dietary PCB 126 on the me-tabolism of the southern catfish caused an increase in the resting metabolic rate of individual fish. At the mitochondrial level of the liver, the stress effects of dietary PCB 126 on the metabolism exhibited that the metabolic rates were increased at a lower level of concentration, there was more energetic expenditure needed by the fish to resist the contaminants, and an increase in capacity of supplying metabolic energy by the liver mitochondria, which should be a physiologically regula-tive mechanism for this fish species to cope with persistent organic pollutants such as PCBs. However, the higher concen-trations of dietary PCB 126 may cause an intolerable functional impairment of mitochondria, so that, it was unable to have a further regulation of the metabolism to the stress of PCB 126.