Present study was conducted to determine the effects of long term carbosulfan exposure on erythrocyte and liver acetylcholinesterase (AChE) activity in rainbow trout (Oncorhynehus mykiss.), and to assess sensitive tissue to carbosulfan in terms of AChE activity. For these purpose, fish were allowed to recover in toxicant-free water for 24 days after 60 days of exposure. AChE activity was determined spectrophotometrically using acetylthiocholine iodide as substrate in the erythrocyte and liver. Erythrocyte and liver AChE of carbosulthn-exposed fish showed considerable inhibition rate. A higher degree of enzyme inhibition was observed in the erythrocyte when compared with liver. The degree of enzyme inhibition had a positive correlation with the time of exposure. Erythrocyte and liver AChE activities were recovered after iS d and 21 d, respectively. Results indicate that the erythrocyte AChE activity is more sensitive to carbosulfan exposure than that of liver AChE. The greater sensitivity of the erythrocyte AChE suggests that it may be more useful as a biomarker of carbosulfan exposure or contamination.