This investigation presents the results of acute toxicity in Clarias gariepinus (C. gariepinus) exposed to median lethal dose of nitrite (58 mg/L) for 1, 2 and 4 days. The 96 hr  LC₅₀ of nitrite for C. gariepinus was determined. Behavioural changes, as well as the developing lesions in nitrite-treated fish were recorded. Hematological changes and chromosomal aberrations induced were also observed. The results revealed that, the nitrite0treated fish developed a significant decrease in erythrocyte population, total leukocyte count, hematocrite value and hemoglobin content. The percent of methemoglobin was highly significant increased in all nitrite-treated groups compared with controls. Serum cortisol was significantly higher in nitrite-treated fish for 4 days compared with control. Different types of chromosomal aberrations were observed as break, fragment, deletion, end to end, endomitosis and centromeric attenuation. The total aberrations were significantly increased in all nitrite-treated fish compared with controls. Addition of sodium chloride (25 mg sod. Chloride/ 1 mg nitrite) or transferring of fish to freshwater after 24 hr of exposure treat methemoglobinemia. Hemoglobin content and methemoglobin precent of recovered fish in both treatments returned nearly to the normal level within 24 hr post treatment.