Abstract: Mosquitofish (-Gambusia affinis-) were exposed to different concentrations of fenitrothion in a static system for 96h. Brain acetylcholinesterase (AChE) activity was evaluated after 24, 48 and 96h insecticide exposure. Fish were also pretreated with piperonyl butoxide (PBO) or triphenyl phosphate (TPP) and then exposed to fenitrothion. Results indicated that fenitrothion produced significant inhibitory effect on AChE ranging from 44.2% to 65.7% at the sublethal concentrations 0.4 mg/L and 1.3mg/L, respectively. Fish pretreated with TPP exhibited decreased AChE activity whilst in PBO pretreated group increased activity was observed compared with those exposed to fenitrothion alone. The pattern of AChE recovery was also assessed in fish previously exposed for 96h and then transferred to clean (chemical free) water. Following 8 days of recovery period, the AChE activity of those exposed to fenitrothion alone and pretreated with TPP was still lower than that of the control.