1-(4-Phenoxyphenoxypropyl)imidazole (KS-175), which has two types of characteristic moieties of insect growth regulators (IGRs), the phenoxyphenoxyalkyl group of juvenile hormone analogs (JHAs) and imidazole of 1,5-disubstituted imidazole such as KK-42, was tested for its biological activity on the silkworm, Bombyx mori. Penultimate (4th) instar larvae topically treated with KS-175 did not molt for more than 20 days. This activity was different from that reported for any IGRs. After the treatment, ecdysteroid levels in the hemolymph did not increase and the cells of the prothoracic gland had shrunk. When the treated penultimate larvae were fed an artificial diet supplemented with 20 ppm of 20-hydroxyecdysone, the larvae molted to the ultimate (5th) instar with a timing similar to that of control larvae fed a diet with or without 20-hydroxyecdysone. These results suggest that topical application of KS-175 irreversibly damages ecdysone biosynthesis in the prothoracic glands.