Two experiments were conducted in this study to determine the effect of ascorbic acid level on the growth performance and pathology of tilapia fish subjected to bacterial infection. Purified diets with six levels (0, 50, 150, 250, 500 & 1000 mg/kg) of supplemental L-ascorbic acid (AA) were pelleted and fed to tilapia fish (20 g) for 12 weeks in each experiment. In the first experiment:- The growth rate of fish fed AA free diet was significantly (P<0.05) lower than those fed on dietary supplemented ascorbic acid. Approximately, 50 mg AA/kg diet was sufficient to prevent the appearance of deficiency symptoms, while a level of 150 mg AA/kg diet significantly (P<0.05) improved final body weight, feed conversion, specific growth rate and protein efficiency ratio. Increasing AA level in the diets significantly (P<0.05) increase the crude protein content of the body, on the contrary to fat content which decreased by the increase of AA level. Maximum mortality was recorded with fish fed on the AA free diet with haemorrhage at the mouth and deformities. The AA contents of the body tissues increased significantly (P<0.05) with increasing AA level up to 500 mg/kg diet. The highest tissue concentration of AA was recorded at the level of 500 mg/kg diet and storage was found to be maximal in the spleen followed by liver, muscle and kidney. In the second experiment:- Tilapia fish were fed purified diets containing AA ranging from 0 (free diet) to 1000 mg/kg until the external signs of deficiency were seen in fish fed AA free diet. At this time, resistance to bacterial infection (Pseudomonas fluorescens) was assessed for fish from the various dietary treatments. Mortality rates of fish experimentally infected with P.fluorescens decreased with increasing the dietary AA doses, ranging from 90% for fish fed the AA-free diet to 10% for fish fed 1000mg AA/kg diet. The haematological parameters were improved and the pathological lesions of bacterial infection were minimized gradually with increasing the ascorbic acid concentration in the diets.