The phenomenon of knockdown resistance (kdr) was first noted in the housefly (Musca domestica), and has subsequently be found (i.e. kdr-type resistance) in several other insect pests including the German cockroach (Blattella germanica). This type of resistance causes insensitivity of the nervous system to pyrethroids, DDT and a limited number of sodium channel neurotoxins. In the German cockroach, kdr-type resistance is incompletely recessive, monogenic and not sex linked or due to cytoplasmic factors. Additionally, kdr-type resistance is not associated with a change in sodium channel density. kdr or kdr-type loci are tightly linked or identical to the para-homologous sodium channel locus in German cockroach, housefly and tobacco budworm (Heliothis virescens), suggesting that kdr and kdr-type resistance are due to mutations in the para-homologous sodium channel gene. kdr-Type resistance in the German cockroach appears similar, although not necessarily identical, to kdr in houseflies.