Susceptibility to permethrin and deltamethrin was assessed in the house fly, Musca domestica L., and blow fly Chrysomya megacephala (F.). The laboratory strain of M. domestica was captured in Muang District (downtown), Chiang Mai, northern Thailand, and the field strains were collected from Muang District, two agricultural areas of Hang Dong District and San Khampaeng District. These strains displayed susceptibility to both permethrin and deltamethrin insecticides, with LD50 values of 0.0049, 0.0019, 0.0003, and 0.0005 microg/fly and 0.1058, 0.0185, 0.1251, and 0.0981 microg/fly, respectively. Tests on the laboratory strain of C. megacephala and the field strains collected from four areas of Chiang Mai (Muang District [downtown], two agricultural areas of Hang Dong District, and San Khampaeng District) and the wooded area near the National Forest revealed slightly lower susceptibility compared with house fly, with LD50 values of permethrin equal to 0.0028,0.0027,0.0024, 0.0007, and 0.0034 microg/fly and of deltamethrin equal to 0.0461, 0.0267,0.1327,0.1151, and 0.0244 microg/fly. These results indicated that the adult populations of both fly species were susceptible to both insecticides.