One version of the Red Queen hypothesis suggests that sexual reproduction is maintained in populations because of the need to continually create genotypes that confer resistance against rapidly evolving pathogens and parasites. Here, we report that parthenogenetic individuals of the Heteronotia binoei species complex are much more prone to infection by mites than are their sexual relatives. This accords with a central prediction of the Red Queen hypothesis. The greater susceptibility of the parthenogens is consistent across localities with different combinations of parthenogenetic genotypes and sexual chromosome races and occurs despite the unusually high genetic diversity of the parthenogenetic form. These observations support the contention that clonal reproduction increases the susceptibility of hosts to infection by parasites.