A field survey was conducted in the summer of 2003 and 2004 to evaluate resistance to deltamethrin on house flies, Musca domestica, from 17 urban garbage dumps in the cities of Beijing, Tianjin, and Zhangjiakou. Bioassays were carried out by topical application with technical deltamethrin to measure the levels of resistance to deltamethrin. Tests for synergism with piperonyl butoxide (PBO) were used to study the occurrence of metabolic resistance mechanisms. Competitive polymerase chain reaction (PCR) amplification of specific allele (cPASA) assay was used on individual house flies from these field populations to detect the presence of the kdr and super-kdr alleles associated with pyrethroid resistance. Another PCR followed by labeled allele-specific oligonucleotide hybridization (PCR-ASO) confirmed the presence of the mutation and hence checked the accuracy of cPASA. Deviation of genotype ratios from Hardy-Weinberg equilibrium was tested. Relative to a susceptible laboratory strain, field house flies from all populations exhibited a 13- to 250-fold greater resistance to deltamethrin in all the populations of house flies at the LD50 and the PBO synergism coefficient was 26- to 124-fold greater in 11 populations collected from Beijing. The super-kdr allele was not found in any of the populations. Results of PCR assays showed that the kdr allele was present at various frequencies in the field populations. ASO-PCR showed that the cPASA method had given only 1 of 33 of false positives. Deviation of frequencies of kdr alleles genotypes from the Hardy-Weinberg ratios was found in the BJF-1, BJF-2, BJF-3, BJHD, and BJXC populations. Regression analysis indicated a significant correlation between kdr allele frequencies and the levels of resistance to knockdown resistance by deltamethrin. The results validate the role of the PCR-based molecular assay as a diagnostic tool in monitoring resistance to pyrethroids and also to provide useful information on population genetics of house fly resistance to pyrethroids.