The circadian clock orchestrates physiological and behavioral activities, including metabolism, neuronal activity, and cell proliferation in synchrony with the environmental cycle of day and night. Here we show that the Drosophila ortholog of the CBP/p300 family of transcription co-activators, nejire (nej), is an intrinsic component of the circadian clock that performs regulatory functions for circadian controlled transcription. Screening of overexpression mutants revealed that gain of nej function was associated with a loss of behavioral and molecular rhythms. Overexpression of NEJ suppresses the long period phenotype of a mutation in the clock gene period (per). NEJ physically interacts through two binding sites with CLOCK and the CLOCK. CYCLE (CLK.CYC) complex. Induction of CLK.CYC-dependent transcripts upon induction of nej expression from a heat-shock promoter showed that NEJ is limiting. Reduced CLK.CYC-mediated transcription in a nej hypomorphic mutant indicates an essential function of NEJ/CBP for CLK.CYC activity and a regulation of circadian transcription by availability of the co-activator. Competition for recruitment of NEJ/CBP provides a potential mechanism for cross-talk between circadian transcription and other CBP-dependent physiological processes.