In Drosophila embryos, graded activity of the posterior determinant nanos (nos) generates abdominal segmentation by blocking protein expression from maternal transcripts of the hunchback (hb) gene. When active inappropriately at the anterior pole, nos can also block expression of the anterior determinant bicoid (bcd). We show that both regulatory interactions are mediated by similar sequences in the 3' untranslated region of each transcript. These nos response elements (NREs) are both necessary and sufficient to confer nos-dependent regulation, the degree of regulation determined by the number and quality of the elements and the level of nos in vivo. Based on these and other results, we argue that nos acts as a morphogen, controlling hb expression (and hence abdominal pattern) as a function of its concentration-dependent interaction with the NREs.