Adverse effects of Allium cepa bulb has been well overlooked because it is natural and assumed safe. This study investigated the lethal doses and histopathological changes in liver and kidney of healthy Clarias gariepinus sub-adult exposed to red Allium cepa bulb at varying concentrations (200, 150, 100, 50 and 25 g/kg) of A. cepa via diet and bath (5, 3, 1.5, 0.7 and 0.4 g/L) for two weeks. Specimens were sacrificed, liver and kidneys collected, processed and examined for histopathological changes. Proximate analysis, qualitative and quantitative phytochemistry was determined using standard methods. Brine Shrimp lethality assay (BSLA), LD₅₀ and LC₅₀ of the onion on experimental fish were investigated. Saponins, tannins, phenols, flavonoids and alkaloids were present. LC₅₀ of the onion extract was between 0. 51 mg and 0.64 mg in the BSLA while mean LD₅₀ and LC₅₀ for dietary and bath treatments were 447.1 g/kg and 12.2 g/L. Liver histopathology showed vacuolar degeneration of the hepatocytes and congestion of central vein, while necrosis of the epithelial cells and haemosiderosis were observed in the kidney at high concentrations. Allium cepa is safe in fish when administered in lower dose either through diet or bath exposure but was toxic at high dose.