This study was designed to elucidate the effect of the organophosphate fenthion exposure on cholinesterase-specific activities in brain, liver, and kidney tissues of juvenile Oreochromis niloticus, and to define the best indicator tissue to fenthion exposure. The 96-h LC(50) value was determined as 2.27 mg/L and fish were exposed to 20% of this concentration for 24-, 48-, and 96-h. Recovery periods in similar durations were provided to evaluate the ChE activities. AChE and BChE activities were determined spectrophotometrically. The activities of these enzymes were significantly inhibited in all the tissues tested, liver was the most and kidney was the least affected tissues. The inhibition percentages of AChE and BChE were at similar levels in the liver while BChE was more affected in kidney. BChE was not detected in the brain. A significant positive correlation in ChE inhibitions was found among tissues, and the effect of fenthion on ChE activities was tissue specific. In general enzymes activities were not significantly recovered in 96-h recovery period; however, an elevation in AChE inhibition was observed in brain. Based on the data of this study, the liver may be suggested as the best indicator tissue especially for phosphorothioate exposure.