BACKGROUND: Common carp (Cyprinus carpio) fishes during release  into the rivers estuary  of the Caspian southern basin are  generally exposed to a broad spectrum of agri-cultural pesticides. OBJECTIVES: The objective of this study was to investigate the effects of three agricultural pesticides including Malathion, Carbaryle and Glyphosate on C. carpio in lethal level by determining LC50 96h and sub-lethal levels via cholinesterase (ChE) activity. METHODS: The median lethal concentration using a standard method which is called  OECD No. 203 (1992), was measured. About 300 fingerlings with average weight of 2.0 ± 0.4 g were randomly selected and were then exposed to  each pesticide in  three treatments (0.1, 0.2 LC50 96h and negative control) in three replications. 5, 10 and 15 days after the test period, sampling from the head and body of fishes was carried out. The ChE activity was assayed with biochemical method described by Ellman. RESULTS: The LC50 96h for three glyphosate, malathion and carbaryle pesticides were obtained as 6.75, 1.3 and 12.67 mg/L, respectively. The mean values of ChE for both head and body under control conditions were found 1241.356 and 723.103 mU/min/mg protein, respectively. Therefore, the ChE activity of head was 1.7 times more than the body. During the test period, inhibition activitiy of ChE was significantly observed in the fishes treated by any of three components in comparison  with control (p<0.05). The ChE inhibition potential by carbaryle and glyphosate was lower than malathion as compared with control. The exposure time concentration exhibited a significant effect compared to  the fishes treated by the investigated pesticide types. CONCLUSIONS: The used pesticide concentrations for non-target species  which were ineffective and permissible according to the lethality bioassay test can lead to their enzyme responses and bio-damages.