The presence of the two mutations T929I-L932F in the voltage-sensitive sodium channel gene associated with permethrin resistance (kdr-like) was shown in head louse, Pediculus humanus capitis De Geer, populations in Denmark. The existence of one susceptible and one T929I-L932F haplotype based on the limited single nucleotide polymorphism (SNP) of these sequences was established. One louse had an SNP causing a G943A substitution in the trans-membrane segment five of domain II on the sodium channel, which has not been identified in other insect species. A polymerase chain reaction-restriction endonuclease method using genomic DNA to discriminate between resistant and susceptible alleles in head lice was developed and implemented. The SNP that results in the T929I substitution also creates a cutting site for the restriction endonuclease SspI, and the presence of one or two SspI cutting sites in head lice is diagnostic for the T929I-L932F haplotype.