Introduction: Chlorpyrifos is an agrochemical pesticide of fame that is commonly used by farmers to control pests. This chemical may become harmful to fish when washed into waterways leading to aquatic environment. This study investigates the histo-morphometric changes in the gills of African Catfish Clarias gariepinus exposed to acute toxic concentrations of chlorpyrifos. Materials and Methods: Clarias gariepinus fingerlings (weight 7.28 ± 0.03g and length 4.82 ± 0.06cm) were exposed to grades of (6.25, 12.5, 25, 50 and 100 µml/L) chlorpyrifos in solution. The 96 hour LC50 value of chlorpyrifos was found to be 160 µml/L. Results: The grades of chlorpyrifos induced several gill histo-architectural damages such as: moderate to severe gill epithelia sloughing, primary and secondary lamellar hyperplasia and central veinous congestion in the parenchyma with pronounced severity in fish exposed to higher concentrations. Similarly, the gill morphometrics (secondary lamellar length, width, interlamellar distance and surface area) were markedly altered by the graded concentrations of chlorpyrifos. Conclusion: Chlorpyrifos appears to be very toxic to fish and therefore, its use near fish farms or in areas close to aquatic environments should be discouraged.