The Staufen-dependent localisation of oskar mRNA to the posterior of the Drosophila oocyte induces the formation of the pole plasm, which contains the abdominal and germline determinants. To identify essential genes required for this process, we performed a germline clone screen for mutations that disrupt the posterior localisation of GFP-Staufen, and isolated three missense alleles in the hnRNPA/B homologue, Hrp48. These mutants specifically abolish osk mRNA localisation, without affecting its translational control or splicing, or the localisation of bicoid and gurken mRNAs and the organisation of the microtubule cytoskeleton. Furthermore, Hrp48 co-localises with osk mRNA throughout oogenesis, and binds directly to its 5' and 3' regulatory regions. Thus, Hrp48 is the first example of a trans-acting factor that binds to oskar mRNA to mediate its posterior transport. The hrp48 alleles cause a different defect in oskar mRNA localisation from all other mutants, and disrupt the formation of GFP-Staufen particles. This defines a novel step in the localisation pathway, which may correspond to the assembly of Staufen/oskar mRNA transport particles.