Localization of the maternally synthesized nanos (nos) RNA to the posterior pole of the Drosophila embryo provides the source for a posterior-to-anterior gradient of Nos protein. Correct spatial regulation of nos activity is essential for normal pattern formation. High local concentrations of Nos protein in the posterior of the embryo are necessary to inhibit translation of the transcription factor Hunchback in this region, and thus permit expression of genes required for abdomen formation (see ref. 5 for review). By contrast, misexpression of Nos protein at the anterior of the embryo prevents translation of the anterior morphogen Bicoid, suppressing head and thorax development. Posterior localization of nos RNA is mediated by sequences within the nos 3' untranslated region (3'UTR) and requires the function of eight genes of the 'posterior group'. Although the unlocalized nos RNA is stable in embryos from females mutant for any of the posterior group genes, these embryos appear to lack nos activity because they develop the abdominal defects characteristic of embryos produced by nos mutant females. We report here that unlocalized nos RNA is translationally repressed. Translational repression is mediated by the nos 3'UTR and can be alleviated either by replacement of the 3'UTR with heterologous 3'UTR sequences or by posterior localization. Thus, RNA localization provides a novel mechanism for translational regulation.