Two classes of models attempt to explain why obligate parthenogenesis only rarely replaces sexual reproduction in natural populations, in spite of the apparent reproductive advantage that parthenogens gain by producing only female offspring1. The mutation-accumulation models suggest that sex is adaptive because it purges the genome of harmful recurrent mutations2,3. The ecological genetic models postulate that sex is adaptive in variable environments, particularly when the relevant variation is generated by coevolutionary interactions with parasites4-7. Both of these models have considerable merit, but would seem to have limitations. The mutation-accumulation models require high rates of mutation3,8; the coevolutionary models require that parasites have severe fitness effects on their hosts9. In addition, parasites could select for clonal diversity and thereby erode any advantage that sex gains by producing variable progeny10. Here we consider the interaction between mutation accumulation and host-parasite cevolution. The results suggest that even moderate effects by parasites combined with reasonable rates of mutation could render sex evolutionarily stable against repeated invasion by clones.