Pyrethroids are an important class of insecticides for controlling insect pests, including the German cockroach. Unfortunately, many insects have developed resistance to pyrethroids. One of the most important mechanisms of resistance is kdr (knockdown resistance) which is characterized by neural insensitivity to pyrethroids and DDT. To investigate whether the voltage-dependent sodium channel is involved in kdr-type resistance in the German cockroach, we isolated a 120 bp DNA fragment of the para-homologous sodium channel gene from German cockroaches. Using this fragment as a probe, we identified a restriction fragment length polymorphism (RFLP) of the para-homologous sodium channel gene between susceptible and kdr-type resistant German cockroaches. RFLP analysis of F2 and backcross cockroach populations (total of 331 individuals) showed that all homozygous resistant individuals had a 3.7 kb EcoRI fragment, all homozygous susceptible individuals had a 3.0 kb EcoRI fragment, and all heterozygous individuals had both 3.7 and 3.0 kb fragments. No recombination was detected between the kdr-type resistance locus and the para-homologous sodium channel gene. This suggests that the kdr-type resistance locus and para-homologous sodium channel gene are identical or tightly linked (< 0.2 cM) in German cockroaches. Our results provide strong evidence that modification of para-homologous sodium channels is associated with kdr-type resistance.