We have examined the position-dependent expression of two segmentation genes that contain a homeo box, even-skipped (eve) and fushi tarazu (ftz). Products encoded by both genes accumulate in a series of seven transverse stripes along the anterior-posterior body axis of developing embryos. Double-staining experiments show that eve and ftz proteins accumulate in complementary sets of embryonic cells. A total of eight zygotically active segmentation genes are required for the establishment and refinement of the eve and ftz expression patterns during development. Mutations that affect one of the genes were always found to produce a reciprocal alteration in the other. These results suggest that the eve and ftz promoters independently "interpret" the same positional cues to give differential patterns of expression. We propose that a combination of segmentation gene products that activates the expression of eve represses the expression of ftz, and vice versa. This proposal is discussed in the context of a hierarchy of interactions among segmentation genes, whereby the gap genes establish asymmetric patterns of eve and ftz expression in blastoderm-stage embryos, and interactions with other pair-rule genes serve to refine further the complementarity of eve and ftz expression during gastrulation and germ band elongation.