We investigated the oxidative stress of methamidophos on Mytilus edulis. Sublethal concentrations of methamidophos (5.0, 10.0 and 15.0 μg L-1) were administered to M. edulis for 5, 15 and 30 days respectively. The biochemical stress response of M. edulis was studied taking into account acetylcholinesterase (AChE), Na+-K+-adenosine triphosphatase (Na+-K+-ATPase) and some antioxidant enzyme activities. Protein contents in tissues was also measured. With the increase of methamidophos concentrations the activities of AChE and Na+-K+-ATPase in tissues decreased, whereas the activities of GPx, SOD and CAT increased. The protein content in tissues decreased after 5 days and then gradually increased until the 15th day of the experiment. On the 30th day, the protein level in organism was observed to have reached to that of the control group. It can be stated that methamidophos exerted its effect even at low concentration , its toxicity increased depending on its dosage and exposure time. This study revealed that M. edulis developed adaptive response to neutralize the oxidative stress following pesticide exposure.