ABSTACT: The microscopic examination of tissue in order to study the manifestations of disease is very important. This paper investigated the histopathological variation in some organs of juveniles of Nile tilapia-Oreohromis niloticus exposed to benzalkonium chloride (BAC)-treated produced water (TPW) mixture in a 96hour acute toxicity test. Fish specimens of average body length and weight 7.80 cm and 25.05g, respectively were used. At the end of exposure 2 survivor fish in each concentration

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... ross triplicates were randomly harvested for the histological examination. Histological preparations of gills, kidney and liver were analysed for histopathological alterations. Results of histological examination revealed various degrees of legions ( gills-cellular necrosis and epithelial rupture (CN), kidney -glomerular expansion (GE) and dilation of Bowman's space (BC) and liver -expanded central vein (CV) and melanomacrophages aggregate (M)) depending on concentrations of the toxicant as against untreated (control) that recorded no abnormality. The results showed that the severity of tissue alterations recorded was a function of toxicity ofbenzalkonium chloride (BAC) mixture with treated produced water (TPW). It is strongly recommended its disposal followed a thorough treatment.