AbstractSeventeen substituted imidazoles were tested as inhibitors of juvenile hormone (JH) III synthesis by cockroach corpora allata in an in‐vitro radio‐chemical assay. Most of these 1,5‐disubstituted imidazoles were highly potent, with IC50values of less than 100nM. The compounds differed in their ability to cause an accumulation of the precursor methyl farnesoate in the glands. Four of the imidazoles were tested by topical application to previtellogenic adult females, and all caused a significant inhibition of JH synthesis and an accumulation of intraglandular methyl farnesoate for at least three days after treatment. Methyl farnesoate epoxidase activity of homogenates of corpora allata was inhibited by the compoundsTH‐14 andTH‐27. This P450‐dependent epoxidase activity was inhibited at less than 10 nM. The results show that the 1,5‐disubstituted imidazoles are powerful inhibitors of the last step of juvenile synthesis in t