The generation of anterior-posterior polarity during development in Drosophila requires the localization of determinant molecules to the anterior and posterior poles of the embryo. Localization of the maternally synthesized nanos RNA to the posterior pole of the embryo is essential to provide a source for a gradient of Nanos protein that directs abdomen formation. nanos RNA localization occurs during oogenesis and requires the function of at least nine genes. cis-acting sequences that direct nanos RNA localization lie within the nanos 3'UTR. In this analysis, we have used nanos 3'UTR deletion mutants to define the localization signal more precisely. Our results indicate that the nanos RNA localization signal is large and complex and that targeting of nanos RNA may be achieved by the combined effects of multiple, partially independent sequences. This idea is supported by evolutionary conservation, both in sequence and in function, of the nanos 3'UTRs of Drosophila melanogaster and Drosophila virilis.