The acute and subchronic toxicity of diazinon and sodium benzoate to the Nile tilapia fish, Oreochromis niloticus L. 60 g weight were studied. The obtained results showed that the LC₅₀ after 24, 48, 72 and 96 hr., post treatment to the insecticide diazinon were 8.377, 6.852, 6.200 and 5.679 mg/l, respectively. Exposed fish to sublethal concentration of diazinon (0.56 mg/l) and sodium benzoate at 150 mg/l for 21days caused some biochemical and histopathological changes in some blood components and organs. Diazinon increased alanine amino transferase (ALT), aspartate amino transferase (AST), glucose, total protein, albumin, creatinine and cholesterol activities, while acetylcholinesterase (AchE) and uric acid were decreased. Sodium benzoate caused an increase of ALT, AST, glucose, total protein, albumin, creatinine, uric acid and cholesterol activities. The histopathological studies showed degeneration of hepatocytes, congestion of blood vessels and revealed degenerative changes on some organs and cystic dilatation of some renal tubules together with congestion of renal blood vessels and mild to moderate lymphocytic infiltration were detected compared with control fish (untreated)