The CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) gene encodes a MYB-related transcription factor involved in the phytochrome induction of a light-harvesting chlorophyll a/b-protein (Lhcb) gene. Expression of the CCA1 gene is transiently induced by phytochrome and oscillates with a circadian rhythm. Constitutive expression of CCA1 protein in transgenic plants abolished the circadian rhythm of several genes with dramatically different phases. These plants also had longer hypocotyls and delayed flowering, developmental processes regulated by light and the circadian clock. Furthermore, the expression of both endogenous CCA1 and the related LHY gene was suppressed. Our results suggest that CCA1 is a part of a feedback loop that is closely associated with the circadian clock in Arabidopsis.