During larval and pupal development of insects, ecdysone is synthesized in the prothoracic gland (PG). Although several Drosophila genes, including Halloween P450 genes, are known to be important for ecdysteroidogenesis in PG, little is known of the ecdysteroidogenic genes in other insects. Here we report on Cyp302a1/disembodied (dib-Bm), one of the Halloween P450s in the silkworm Bombyx mori that is a carbon-22 hydroxylase. dib-Bm is predominantly expressed in PG and its developmental expression profile is correlated with a change in the ecdysteroid titre in the haemolymph. Furthermore, dib-Bm expression in cultured PGs is significantly induced by treatment with prothoracicotropic hormone. This is the first report on the transcriptional induction of a steroidogenic gene by the tropic hormone in insects.