Many organisms use circadian clocks to keep temporal order and anticipate daily environmental changes. In Drosophila, the master clock gene Clock promotes the transcription of several key target genes. Two of these gene products, PER and TIM, repress CLK-CYC-mediated transcription. To recognize additional direct CLK target genes, we designed a genome-wide approach and identified clockwork orange (cwo) as a new core clock component. cwo encodes a transcriptional repressor that synergizes with PER and inhibits CLK-mediated activation. Consistent with this function, the mRNA profiles of CLK direct target genes in cwo mutant flies manifest high trough values and low amplitude oscillations. Because behavioral rhythmicity fails to persist in constant darkness (DD) with little or no effect on average mRNA levels in flies lacking cwo, transcriptional oscillation amplitude appears to be linked to rhythmicity. Moreover, the mutant flies are long period, consistent with the late repression indicated by the RNA profiles. These findings suggest that CWO acts preferentially in the late night to help terminate CLK-CYC-mediated transcription of direct target genes including cwo itself. The presence of mammalian homologs with circadian expression features (Dec1 and Dec2) suggests that a similar feedback mechanism exists in mammalian clocks.