Abstract: The results of typical application tests showed that the oriental housefly ( Dom-estica vicina ) in the northern suburb of Tianjin city has developed resistance to DD-VP. In 1983, the LD₅₀ value of the housefly was 0.473 μg, 35 times larger than that of the susceptible strain. In 1986, the resistance rate, compared with that in 1983, dereased by 14 times. As to pyrethroid phenothrin no resistance has yet been found in that housefly.The bimolecular rate constant ( Ki ) for the reaction of DDVP with the housefly head AchE has been determined as 0.7×10⁵ mol^-1 min^-1. The resistance capacity of oriental housefly to DDVP was probably due to an altered form of AchE which showed a greatly reduced rate of reaction with inhibitor.