The present study was conducted to clarify what occurs during the metamorphosis of the imaginal disc in insects. To understand the metamorphosis on a molecular level, the changes in expression profiles in the imaginal disc during metamorphosis were investigated. For this purpose, we constructed cDNA libraries from four different stages of wing discs of Bombyx mori, sequenced about 1000 cDNAs randomly collected from each library, and constructed a database of expressed sequence tags (EST). The morphological changes and expression profiles from EST were compared during those four stages. Microarray analysis was applied to quantify the expression of each gene in each stage in order to confirm whether the expression of the genes identified from EST was induced by 20-hydroxyecdysone (20E) in a stage-specific manner. Wing discs showed dynamic morphogenesis in 4-5 days during the preparatory stage of metamorphosis, which was under the control of an ecdysteroid. Different expressed profiles were observed in each of the four different stages by comparison of each EST clone. These profiles reflected the morphological changes of the Bombyx wing disc during metamorphosis. The results of expression profiles from the four stages suggested that the V4 stage was cell proliferating; W0, proliferating and the beginning of differentiation; W2, morphologically changing; W3, cuticle secreting. Microarray analysis showed the effectiveness of its application on 20E induction of genes in wing discs. The wing disc of B. mori is an exceptionally suitable system for understanding the relationship between morphological changes and the distribution of mRNA.