In the present study, a permethrin-resistant strain (ALHF) of housefly was used to understand some enzymic changes in normal biosynthetic pathways after insecticide selection. Aflatoxin B(1) (AFB(1)) as a natural substrate was used to verify the changes on the level of cytochrome P450-dependent monooxygenases and oxido-reductase activities in the ALHF strain compared to an insecticide-susceptible strain, aabys. ALHF yielded three major biotransformation products: aflatoxin B(2a) (AFB(2a)), aflatoxin M(1) (AFM(1)), and aflatoxicol (AFL) by larvae. These principal products were also found in aabys. AFL production rate of ALHF larvae was 5-fold lower than that of aabys. Differences between ALHF larvae and aabys in AFM(1) production were found. ALHF did not differ significantly from aabys in AFB(2a) production. The levels of 17α- and β-hydroxysteroid dehydrogenase (17α- and β-HSD) were also determined to elucidate which type of dehydrogenase activities could be changed. The cytosolic fraction of ALHF larvae yielded about 2-fold higher 17α-estradiol than that of aabys larvae. In contrast, the microsomal fraction of ALHF larvae produced about 2-fold lower amount of 17α-estradiol than that of aabys larvae. The production rate of microsomal fraction of 17β-estradiol ALHF larvae yielded 3-fold lower than that of aabys larvae. Inhibition studies on 17α-HSD and 17β-HSD activities by pyrethroid insecticides showed that there was no inhibition by pyrethroids on the enzyme activity. Therefore, there seems to be no changes on the enzyme structures. Changes on enzyme expression may occur in ALHF larvae in relation to 17α- or β-HSD. To assess biochemical changes of the cuticle formation phenylalanine 4-hydroxylase and tyrosinase activities were determined. The production rate of tyrosine from phenylalanine in ALHF was about 2-fold higher for larvae than that in aabys. l-(dihydroxylphenyl)alanine (DOPA) content was determined in larvae and ALHF possessed 1.6-fold larger amounts of DOPA than aabys. Tyrosinase activity of ALHF larval preparations showed 1.6-fold higher than aabys. In summary, many enzymic changes were found in ALHF strain compared to aabys strain and these changes may be resulted from the permethrin selection.