ABSTRACT Establishment of the anteroposterior and dorsoventral axes in the Drosophila egg chamber requires reciprocal signaling between the germ line and soma. Upon activation of the Drosophila EGF receptor in the posterior follicle cells, these cells signal back to the oocyte, resulting in a reorganization of the oocyte cytoplasm and anterodorsal migration of the oocyte nucleus. We demonstrate that the gene mago nashi (mago) encodes an evolutionarily conserved protein that must be localized within the posterior pole plasm for germ-plasm assembly and Caenorhabditis elegans mago is a functional homologue of Drosophila mago. In the absence of mago+ function during oogenesis, the anteroposterior and dorsoventral coordi-nates of the oocyte are not specified and the germ plasm fails to assemble.