During somitogenesis, cells are recruited to the caudal presomitic mesoderm (PSM) from the primitive streak (and later the tail bud), while somites separate from the rostral end as epithelial cubes [1]. This is a regular process, one somite forming every 2 hours in the mouse, that can be simulated by clock and wavefront models [2]. The chick basic helix-loop-helix transcription factor encoded by c-hairy1 is expressed in dynamic waves in the PSM, undergoing one cycle for each somite formed [3]. This is compatible with an underlying oscillating molecular clock. We have shown here that Lunatic fringe (L-fng) expression is indicative of it being one of the implementing outputs of this clock. Fringe genes regulate the Notch signalling pathway in boundary formation [4,5]. Of the known mouse genes, only L-fng is expressed in PSM [4,5] and it is required for somite segmentation and patterning [6,7]. We have now shown that L-fng is expressed as dynamic, repetitive and complex waves within the mouse PSM. A wave takes 4 hours to complete one cycle and terminates immediately at, and prior to, somite boundary formation. Consecutive waves are temporally but not spatially overlapping, being initiated in the caudal PSM every 2 hours, so offset by one half-cycle. Waves of expression are not associated with cell movement and do not require cell contact for propagation, so appear to reflect a cell-autonomous clock that is synchronous in all PSM cells.