AbstractStudents of insect metamorphosis have in the past accepted the paradigm that the structures of different metamorphic stages are built from molecules coded for by different sets of genes. A substantial body of evidence, based on cuticular proteins, has now accumulated that casts doubt on that paradigm. This paper summarizes the evidence and provides a new framework for viewing the activities of genes during metamorphosis.Studies on promoter utilization for alcohol dehydrogenase inDrosophila melanogasterreveal that cells change promoters at the onset of metamorphosis. These data suggest that modifications of regulatory elements rather than duplications of structural genes may have occurred to permit the evolution of metamorphosis.