Mutants of the maternal posterior-group genes of Drosophila lack posterior body pattern elements and germ cells, both of which form through the action of determinants localized to the posterior pole of the oocyte. We report that transcripts of one of these genes, oskar, become localized to the posterior pole of oocytes shortly after the oocyte begins to differentiate visibly. Analysis of various posterior-group mutants reveals that localization of oskar mRNA is an early step in the posterior localization pathway. In addition, we find that nonsense oskar mutations disrupt osk mRNA localization, while missense oskar mutations do not.