Most organisms have circadian docks consisting of negative feedback loops of gene regulation that facilitate adaptation to cycles of light and darkness. In this study, CRYPTOCHROME (CRY), a protein involved in circadian photoperception in Drosophila, is shown to block the function of PERIOD/TIMELESS (PER/TIM) heterodimeric complexes in a light-dependent fashion. TIM degradation does not occur under these conditions; thus, TIM degradation is uncoupled from abrogation of its function by light. CRY and TIM are part of the same complex and directly interact in yeast in a light- dependent fashion. PER/TIM and CRY influence the subcellular distribution of these protein complexes, which reside primarily in the nucleus after the perception of a light signal. Thus, CRY acts as a circadian photoreceptor by directly interacting with core components of the circadian dock.