Light is an important environmental signal that governs plant growth and development. One important light-signalling component involved in plant light responses is COP1. The pleiotropic phenotypes of the cop1 mutant suggest that COP1 regulates not only photomorphogenesis, but also other developmental processes. We investigated the role of COP1 by identifying genes that are regulated by COP1. We report that AtMYB21, a gene encoding a flower-specific transcription factor, is ectopically expressed in the cop1 mutant. Analysis shows that dark-grown transgenic seedlings expressing AtMYB21-GR fusion protein display some features of the cop1 mutant, including decreased hypocotyl cell expansion, open cotyledons in the dark, and seedling lethality in the presence of dexamethasone. Light-grown adult transgenic plants expressing AtMYB21 have shorter stems, smaller and narrower leaves, narrower petals, and malformed carpels. In addition, we show that AtMYB21 directly regulates two genes that are also expressed more abundantly in the cop1 mutant. The results indicate that COP1 is required to repress the AtMYB21 gene in seedlings, and the pleiotropic phenotypes shown in the cop1 mutant are due to the combination of misregulation of genuine light-signalling components and other tissue-specific factors.