Many aspects of normal retinal physiology are controlled by a retinal circadian clock. In Xenopus laevis, the photoreceptor cells within the retina contain a circadian clock that controls melatonin release. In this report we present the cloning and characterization of the Xenopus homolog of the Clock gene, known to be critical for normal circadian behavioral rhythms in the mouse. The Xenopus Clock gene is expressed primarily in photoreceptors within the eye and is expressed at constant levels throughout the day. Analysis of other tissues revealed that, as in other species, the Xenopus Clock gene is widely expressed. This characterization of the Clock gene provides a useful tool for further exploration of the role of the circadian clock in normal retinal function.