In mammals, sex differentiation of primordial germ cells (PGCs) is determined by extrinsic cues from the environment(1). In mouse female PGCs, expression of stimulated by retinoic acid gene 8 (Stra8) and meiosis are induced in response to retinoic acid provided from the mesonephroi(2-5). Given the widespread role of retinoic add signalling during development(6,7), the molecular mechanisms that enable PGCs to express Stra8 and enter meiosis in a timely manner are unknown(2,8). Here we identify gene-dosage-dependent roles in PGC development for Ring1 and Rnf2, two central components of the Polycomb repressive complex 1 (PRC1)(9,10). Both paralogues are essential for PGC development between days 10.5 and 11.5 of gestation. Rnf2 is subsequently required in female PGCs to maintain high levels of Oct4 (also known as Pou5f1) and Nanog expression(11), and to prevent premature induction of meiotic gene expression and entry into meiotic prophase. Chemical inhibition of retinoic acid signalling partially suppresses precocious Oct4 downregulation and Stra8 activation in Rnf2-deficient female PGCs. Chromatin immunoprecipitation analyses show that Stra8 is a direct target of PRC1 and PRC2 in PGCs. These data demonstrate the importance of PRC1 gene dosage in PGC development and in coordinating the timing of sex differentiation of female PGCs by antagonizing extrinsic retinoic add signalling.