The mechanism of diazinon resistance in the Hokota strain, which was the first organophosphorus (OP) insecticide resistant houseflies reported in Japan in 1961, was investigated using 32P-diazinon and -diazoxon in vitro and in vivo. It was demonstrated that this strain has three resistant mechanisms: slow penetration of diazinon through the cuticle, high microsomal mixed function oxidase activity and high activity of phosphatase which hydrolyzes diazoxon, or an active metabolite of diazinon. About five years after the use of insecticide for housefly control had been completely stopped in order to sericulture, houseflies were collected from the same farm where the diazinon resistant Hokota fly had been found. Single pair culture of captured flies and a diazinon susceptibility test of the offspring showed that a high frequency of diazinon resistant genes was still present. The Akita-f was a strain subjected to selection with fenitrothion for 17 generations. Subsequent evaluation showed the resistance ratio of this then established fenitrothion resistant strain to susceptible SRS strain was about 4000. Crossing between Akita-f and SRS strains showed the fenitrothion resistance almost completely dominant. Linkage group analyses for the dominant factor were carried out using two multi-chromosomal marker strains by the F1 male backcross method, and the dominant resistant factor(s) was found to be associated only with the 2nd chromosome. The Wakamatsu-m strain, highly resistant to malathion, was established from a colony collected in Kitakyushu after more than 10 generations of selection with this chemical. The dominant factor(s) for malathion resistance was also located only on the 2nd chromosome. This strain possesses as its mechanisms of resistance high malathion carboxylesterase activity, high glutathione S-transferase activity and insensitive acetylcholinesterase. The linkage group of the dominant factor controlling high malathion carboxylesterase was analyzed. Results of the crossing experiments clearly showed that only the 2nd chromosome was associated with the dominant factor controlling high malathion carboxylesterase. Forty-two insect metabolites of trans- and cis-permethrin were identified in in vivo studies with American cockroaches, houseflies and cabbage loopers. The permethrin isomers were metabolized by hydrolysis and hydroxylation at the geminal-dimethyl group and the phenoxybenzyl group. The alcoholic and phenolic metabolites are excreted as glucosides, and carboxylic acid is excreted as glucosides and amino conjugates. The microsomal system was used for the metabolism of permethrin isomers by housefly and cabbage looper preparations. Esteric cleavage is more extensive for trans-permethrin than for cis-permethrin, while the relative extent of oxidative metabolism of the two isomers depends on the enzyme source. The Danish pyrethroid resistant housefly strain, 228e2b, showed cross resistance to all pyrethroid insecticides tested. Genetic study revealed that the factor resistant to permethrin was almost completely recessive and was located on the 3rd chromosome. A strain termed PyR which showed extreme resistance to pyrethroid insecticides was obtained after mating the 228e2b strain and Japanese OP resistant Wakamatsu-m strain, and after selections of their offspring by permethrin for 16 generations. In addition to nerve insensitivity, the PyR strain possessed, as a mechanism of resistance, high enzyme activity of microsomal oxidase which contributed to the detoxication of pyrethroids. Major pyrethroid resistance genes in PyR houseflies were located on the 2nd and 3rd chromosomes.