685TOXICITY EFFECTS OF FENITROTHION ON ESTERASE IN TILAPIA FISH (Oreochromis niloticus)This study focused on toxicity effects of fenitrothion on esterase in tilapia fish (Oreochromis niloticus). Specific esterase activity, I 50 value and k i value of O. niloticus were determined. The optimum parameters for esterase activity in O. niloticus were at pH 7 of phosphate buffer, 12x10 -4 M of substrate concentration, at 25 ο C after incubated for 30 minutes. The specific esterase activity was 3.578x10 -7 µmol mg -1 min -1 . The I50 value for fenitrothion was found at 1x10 -4.315 M with the inhibition rate of 14.66. Fenitrothion caused moderate toxic to the population of fishmam.qaqssim@gmail.comResearch6112015Fenitrothion,toxicity,Tilapia fish,Inhibition,Esterase activityAyuni N,Abd Elgadir M,Wan Rozianoor MHPharmacology and Toxicology Laboratory, Faculty of Pharmacy, Universiti Teknologi Mara, 42300 Bandar Puncak Alam, Selangor, Malaysia,Department of Food Science & Human Nutrition, College of Agriculture &Veterinary Medicine, Qassim University, AlQassim, P.O Box 6622 Buraidah 52452, Kingdom of Saudi Arabia,Faculty of Applied Sciences, Universiti Teknologi MARA, 40450, Shah Alam, Selangor, Malaysia