Although mutations in the segment polarity genes wingless, engrailed, hedgehog, gooseberry and cubitus-interruptusD all affect the region of naked cuticle within each segment of the Drosophila larva, subtle phenotypic differences suggest that these genes play different roles in segmental patterning. In this paper, the regulative interactions between these genes are analysed. They have revealed that the products of most of these genes accomplish more than one function during embryogenesis. Whereas early on a positive feed-back loop involving wg, en and hh maintains the expression of wg and en in the extremes of each parasegment, later on wg and en become independent from each other. en appears to regulate the expression of hh and ptc, while wg depends on gsb and ciD.