CLOCK (CLK) is a core component of the transcriptional feedback loops that comprise the circadian timekeeping mechanism in Drosophila. As a heterodimer with CYCLE (CYC), CLK binds E-boxes to activate the transcription of rhythmically expressed genes within and downstream of the circadian clock, but this activation unexpectedly occurs at times when CLK is at its lowest levels on Western blots. Recent studies demonstrate that CLK also regulates nonrhythmic gene expression and behaviors. Despite the critical roles CLK plays within and outside the circadian clock, its spatial expression pattern has not been characterized. Using a newly developed CLK antibody, the authors show that CLK is coexpressed with PERIOD (PER) in canonical oscillator cells throughout the head and body. In contrast to PER, however, the levels of CLK immunoreactivity do not cycle in intensity, CLK is detected primarily in the nucleus throughout the circadian cycle, and CLK is expressed in nonoscillator cells within the lateral and dorsal brain, including Kenyon cells, which mediate various forms of learning and memory. These results indicate that constitutive levels of nuclear CLK regulate rhythmic transcription in circadian oscillator cells and suggest that CLK contributes to other behavioral processes by regulating gene expression in nonoscillator cells.