The oocyte is a unique cell type that undergoes extensive chromosome changes on its way to fertilization, but the chromatin determinants of its fate are unknown. Here, we show that Polycomb group (PcG) proteins of the Polycomb repressive complex 2 (PRC2) determine the fate of the oocyte in Drosophila. Mutation of the enzymatic PRC2 subunit Enhancer of zeste (E(z)) in the germline abolishes spatial and temporal control of the cell cycle and induces sterility via transdetermination of the oocyte into a nurse-like cell. This fate switch depends on loss of silencing of two PRC2 target genes, Cyclin E and the cyclin-dependent kinase inhibitor dacapo. By contrast, the PRC1 component Polycomb (Pc) plays no role in this process. Our results demonstrate that PRC2 plays an exquisite role in the determination of the oocyte fate by preventing its switching into an endoreplicative program.