The establishment of the segmental body pattern of Drosophila requires the coordinated functions of three classes of zygotically active genes early in development. We have examined the effects of mutations in these genes on the spatial expression of the fushi tarazu (ftz) pair-rule segmentation gene. Mutations in four gap loci and in three pair-rule loci dramatically affect the initial pattern of transverse stripes of ftz-containing nuclei. Five other pair-rule genes and several other loci that affect the larval cuticular pattern do not detectably affect ftz expression. No simple regulatory relationships can be deduced. Rather, expression of the ftz gene depends upon the interactions among the different segmentation genes active at each position along the anterior-posterior axis of the early embryo.