Abstract: This study was conducted to investigate the effects of rearing water temperatures on growth performance, lipid metabolism, and antioxidant capacity of juvenile largemouth bass (Micropterus salmoides). Juvenile fish initial mean body weight: (15.00±0.91) g were randomly allocated to five temperature treatments (18, 21, 24, 27, and 30℃) for 30 days, after which an ammonia nitrogen stress test was conducted for 204h. Ammonium chloride stock solution (200 mg/mL) was added to each tank to achieve a total ammonia nitrogen concentration of 64 mg/L at pH 7.5, yielding un-ionized ammonia concentrations of 0.770, 0.957, 1.184, 1.457, and 1.784 mg/L for the respective temperature groups. The results showed that final mean body weight, weight gain rate, specific growth rate, and condition factor increased initially and then decreased with increasing water temperature, peaking at 27℃ and significantly higher than those in other groups (P<0.05). Feed conversion ratio in the 18℃ group was significantly higher compared with other temperature groups (P<0.05). Regression analysis based on weight gain rate indicated that the optimal rearing water temperatures for growth was 25.46℃. Intestine trypsin and lipase activities in the 18℃ group were obviously higher compared with all other temperature groups (P<0.05), whereas no significant differences were observed in intestine amylase activity among groups (P>0.05). Regarding antioxidant capacity, hepatic total superoxide dismutase (T-SOD) activity decreased with increasing water temperature (P<0.05), whereas the activities of glutathione peroxidase (GPx) and glutathione S-transferase (GST) were significantly elevated in the 30℃ group (P<0.05). Hepatic triglyceride content, total cholesterol content, and percentage of Oil Red O staining area were significantly elevated in the 21℃ group, accompanied by significant upregulation of the fatty acid synthesis gene (acc) and fatty acid uptake gene (fatp1) (P<0.05). The 18℃ group exhibited significantly lower hepatic triglyceride content compared with the other groups (P<0.05). The 24℃ and 27℃ groups exhibited elevated expression of apob, a core apolipoprotein gene involved in the assembly and secretion into the blood. The 30℃ group exhibited the lowest triglyceride (TG) and total cholesterol (TC) contents in both serum and liver. No significant differences were observed in the expression of lipid catabolism genes (lpl and cpt1) among groups (P>0.05). Under ammonia nitrogen stress, mortality occurred within 12h after exposure in the 30℃ group, whereas fish in the 18℃ and 21℃ groups survived until 48h, indicating a synergistic toxic effect between elevated temperature and ammonia. Collectively, these results indicate that the optimal growth temperature for juvenile largemouth bass is 25.46℃. Low temperature impairs growth and leads to aberrant hepatic lipid metabolism, whereas high temperature inhibits growth, causes oxidative stress, reduces lipid deposition, and exacerbates the toxicity of ammonia nitrogen stress.