Abstract: For the purpose of finding the toxic effects and mechanism of commonly used pesticide deltamethrin on the red swamp crayfish Procambarus clarkia,the ultrstructrure damages of the muscle cell of Procambarus clarkia were observed by the transmission electron microscope and the cytochrome c oxidase(CCO),the lactic dehydrogenase(LDH)and lactic acid(LD)were used as biomarkers to evaluate the effects of deltamethin on the muscle cell respiration capacity after 24 hour-exposure to 0.5,0.1 and 0.005 μg/L deltamethrin.The results showed that muscle cell ultrastructure injuries were observed under the exposure of the three concentrations of deltamehtrin,with the main characteristics of the myofibrillar fragmentation,sarcoplasmic reticulum dissolution and mitochondria dissolution.Besides that,the three exposure concentrations of deltamethrin all put impacts on the CCO activity,LDH activity and LD amount.The 0.1 μg/L deltamethrin greatly inhibited the CCO activity within 6-24 h exposure process(P<0.01),while the LDH acticity and the LD amount were significantly increased.This result demonstrated that 0.5,0.1 and 0.005 μg/L deltamethrin could cause the ultratructure damages of the muscle cell of Procambarus clarkia and activate the anaerobic respiration which can produce high amounts of LD.Consequently,the accumulation of LD caused the acidification of the muscle cell which will make toxicity get heavier.