This study was conducted to evaluate the effects of an organic insecticide, spinosad on acetylcholinesterase (AChE, EC 18.104.22.168) specific activities in the brain and the liver tissues of juvenile Oreochromis niloticus, and also identify the indicator tissues in the fish. The fish were exposed to three sublethal spinosad concentrations (25, 50, 75 mg/L) for 24-48-72 h. Acetylcholinesterase activities were determined by spectrophotometrical methods. Acetylcholinesterase was significantly inhibited in both tissues tested. The inhibition percentages of AChE ranged for liver and brain tissues between 32-63% and 21-35%, respectively. The present study demonstrated that in vivo spinosad exposure caused AChE inhibition in the brain and the liver. The liver tissue might be suggested as an indicator tissue for spinosad exposure in the fish. Additional studies are needed to understand inhibition mechanisms of AChE by spinosad. (C) 2012 Elsevier B.V. All rights reserved.