This investigation was performed to demonstrate the effect of Saturn herbicide on Nile tilapia (Oreachromis niloticus). Experimental studies were done to determine of LC50 of Saturn and evaluate its acute and chronic toxicity (1/2 LC₅₀ for 7 days and 1/10 LC₅₀ for 60 days). The pathological changes were recorded. The residual contents of the fish tissues (the gills, brain, skin, muscles and ovaries) were estimated. The clearance test was studied by transferring some of chronic Saturn-exposed fish in clear water for 21 days. From the obtained results LC₅₀ after 96 hrs. for Saturn was 8.2 mg/L. Circulatory and degenerative as well as necrotic changes were seen together with tissues residues in the gills, brain, skin, muscles and ovaries. These residues were decreased and some of the histological alterations observed were completely restored. The brain revealed perivascular melanomacrophages proliferation after the recovery period (21days). We can conclude that the investigated Saturn herbicide induced a negative impact on the quality of Nile tilapia as manifested by the marked changes in fish morphology, tissue structures and residues.