Abdominal segmentation of the Drosophila embryo requires the activities of the gap genes Krüppel (Kr), knirps (kni), and tailless (tll). They control the expression of the pair-rule gene hairy (h) by activating or repressing independent cis-acting units that generate individual stripes. Kr activates stripe 5 and represses stripe 6, kni activates stripe 6 and represses stripe 7, and tll activates stripe 7. Kr and kni proteins bind strongly to h control units that generate stripes in areas of low concentration of the respective gap gene products and weakly to those that generate stripes in areas of high gap gene expression. These results indicate that Kr and kni proteins form overlapping concentration gradients that generate the periodic pair-rule expression pattern.