The widespread existence of sexual reproduction is widely considered to one of the most pressing anomalies for evolutionary theory. One possible solution is that coevolution between hosts and parasites can favor sexual over asexual reproduction (the Red Queen hypothesis), provided infection is genotype specific and highly virulent. In this talk, I present evidence from a freshwater snail that bears on the Red Queen hypothesis. The snail has coexisting sexual and asexual forms throughout New Zealand. It is also infected by many different species of digenetic trematodes, which cause rapid sterilization of infected individuals. Recent field studies on this snail show rapid turnover of common clones in mixed (sexual and asexual) populations. In addition, clones that were common in the past became more susceptible to infection over time, suggesting rapid evolution by the parasite. This same result was observed in a controlled experiment, wherein an initially common and resistant clone became more susceptible to infection over several generations, and was driven down in frequency. Finally, recent experimental data show that the shallow-water margins of lakes, where sexual reproduction in the snail is most common, are also coevolutionary “hotspots” for the host-parasite interaction. Taken together, these results suggest that rapid parasite coevolution could help to prevent the fixation of asexual mutants.