This research was conducted to determine the lethal concentration (LC50) of benzalkonium chloride (BAC) mixture with treated produced water capable of causing mortality of at least 50% of the Nile-tilapia-Oreochromis niloticusjuveniles in a static 96h acute toxicity. Juveniles of mix sexes of O. niloticus of average length (6.80 cm) and weight (25.05g) were exposed to various concentrations (20.20, 50.50, 101.00 and 202.00 ppm.) of the toxicant with tap water used as control under laboratory

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... dition. The LC50 values at 24, 48, 72 and 96hours were determined to be 96.427, 73.799, 73.799 and 69.772 ppm, respectively. Water quality check and general fish behaviour (respiratory distress, loss of balance, erratic swimming and death) were observed. Regression equation(y =-0.270 + 93.44) and correlation (R 2 = 0.751) obtained from the study depict direct positive relationship between fish mortality and BAC mixture. Hence, unregulated release of benzalkonium chloride and treated produced water into aquatic environment is capable of causing acute toxicity to juveniles of O. niloticus. Its disposal into the environment should therefore be discouraged.