|Abstract||The present study examines the level of permethrin and malathion resistance in human head (Pediculosis capitis) and body (Pediculosis humanus L.) louse populations. Putatively insecticide-susceptible body and head louse populations were obtained from Israel (IS-BL) and Panama (PA-HL), respectively. Putatively insecticide-resistant populations of head lice were obtained from Florida (FL-HL) and Massachusetts (MA- HL), in the United States, and from Bristul (BR-HL), in the United Kingdom. Using knockdown and mortality bioassays, we confirmed permethrin resistance in the FL-HL and MA-I-IL populations. Resistance levels of the FL-HL and MA-HL populations were 132 and 223 times higher than that of the IS-BL population and 41 and 68 times higher than that, of the PA-HL population when determined by knockdown resistance ratios. Resistance levels of the FL-HL and MA-HL populations were 4.3 and 4.9 times that of the IS-BL population and 3.8 and 4.3 times that of the PA-HL population when determined by mortality resistance ratios. The significantly higher levels of resistance determined using the knockdown ratios compared to the mortality ratios suggest that nerve insensitivity, most likely mediated by mutations within sodium channel genes, ia a principal mechanism associated with resistance. None of these populations were resistant to malathion. The mortality response was significantly synergized by piperonyl butoxide in the PA-I-IL and MA-I-IL populations, indicating a possible role of oxidative metabolism in these strains. This apparent increase in oxidative metabolism is likely responsible for the lower permethrin sensitivity of the PA-HL versus the IS-BL and the MA-HL versus the FL-HL populations, respectively. Using a homology probing strategy, we have amplified para-orthologous louse sodium channel alpha- subunit cDNA fragments that span the similar to IIS4-IIS6 region, where most of the sodium channel gene mutations that are known to be associated with knockdown resistance are located. Through molecular cloning and sequencing, we have identified two pi,int mutations, T929I and L932F, located in the IISS transmembrane segment of cDNAs from only the permethrin-resistant FL-HL and BR-HL populations. Using homologous primers from our initial sequencing analysis of the cDNA fragments, we obtained a 568-bp genomic DNA fragment and confirmed the presence of the two mutations associated with permethrin resistance. The availability of PCR primers that directly amplify the genomic DNA fragment containing the mutation sites that are widely found associated with knockdown resistance will greatly facilitate the development of DNA-based diagnostic protocols For use in monitoring programs. (C) 2000 Academic Press.