Proper timing of gene expression is essential for many biological events, but the molecular mechanisms that control timing remain largely unclear. It has been suggested that introns contribute to the timing mechanisms of gene expression, but this hypothesis has not been tested with natural genes. One of the best systems for examining the significance of introns is the oscillator network in the somite segmentation clock, because mathematical modeling predicted that oscillating expression depends on negative feedback with a delayed timing. The basic helix–loop–helix repressor gene Hes7 is cyclically expressed in the presomitic mesoderm (PSM) and regulates the somite segmentation. Here, we found that introns lead to an ∼19-min delay in the Hes7 gene expression, and mathematical modeling suggested that without such a delay, Hes7 oscillations would be abolished. To test this prediction, we generated mice carrying the Hes7 locus whose introns were removed. In these mice, Hes7 expression did not oscillate but occurred steadily, leading to severe segmentation defects. These results indicate that introns are indeed required for Hes7 oscillations and point to the significance of intronic delays in dynamic gene expression.