Abstract: The changes of the activity and susceptibility of acetylcholinesterase (AChE) and the changes of the content of protein of susceptible housefly (Musca domestica Vicina L.) after emergence were investigated systemically, and the effects of different pH values on the activity of AChE were also studied. The results showed that, the specific activity of AChE in the head increased gradually from the first day to the fifteenth day after emergence, while the content of protein decreased gradually (187.022 2±9.021 3~83.022 2±7.111 1 μg/head). The susceptibility of AChE in the head increased to highest level from the second day to the fourth day after emergence, and the I50 of dichlorvos of the first day and the fifteenth day after emergence was 1.4 times and 2.1 times compared to that of the third day,respectively. The specific activity of AChE in the head increased with the increasing of the pH value within some scope. These results offered an optmum conditions for detecting phosphorus pesticide residue using AChE from housefly head.