In certain extraembryonic tissues of normal female mouse conceptuses, X-chromosome-dosage compensation is achieved by preferential inactivation of the paternally derived X. Diploid parthenogenones have two maternally derived X chromosomes, hence this mechanism cannot operate. To examine whether this contributes to the inviability of parthenogenones, XO and XX parthenogenetic eggs were constructed by pronuclear transplantation and their development assessed after transfer to pseudopregnant recipients. In one series of experiments, the frequency of postimplantation development of XO parthenogenones was much higher than that of their XX counterparts. This result is consistent with the possibility that two maternally derived X chromosomes can contribute to parthenogenetic inviability at or very soon after implantation. However, both XO and XX parthenogenones showed similar developmental abnormalities at the postimplantation stage, demonstrating that parthenogenetic inviability is ultimately determined by the possession of two sets of maternally derived autosomes.