The effects of Lead Nitrate on catalase (CAT) production levels in post juvenile Clarias gariepinus were investigated. A total of 250 samples of the fish were acclimatized for two week. Six samples of the fish were exposed to the sub-lethal concentrations of 00, 28, 43 and 57 mg/L with replicate in each case for 28 days. Two samples of the fish were randomly selected from the treatments and control for the bioassay of catalase on the 14 th and 28 th day respectively. The fish organs (kidney, liver and gill) were excised from the fish and homogenized in sodium phosphate buffer (0.2 M, pH 8.0). The data generated were subjected to one-way analysis of variance followed by Duncan Multiple Range test where significant. The results showed that 96 h LC 50 of C. gariepinus exposed to lethal concentrations of Lead Nitrate was 284.189 mg/L. The CAT production levels were significantly higher in the kidney of the fish exposed in 28 mg/L on day 14; 43 and 57 mg/L on day 28, respectively. The CAT production levels were significantly higher in the gill of the fish exposed in 57 mg/L on day 14; 43 and 57 mg/L on day 28 respectively. The CAT production levels were significantly higher in the liver of the fish exposed in 28 and 57 mg/L on day 14 respectively. The highest production mean values of 149.55±43.65 and 152.80±40.40 U/mgprotein were obtained in the kidney of the fish exposed in 57 and 43 mg/L, respectively. Therefore, the kidney of the fish exhibited a better control of the toxicant and as such, catalase production level in this organ should be used in assessing the level of physiological changes in the fish.