We measured infection rates of a putative parasitic castrating ciliate, in a population of the obligately parthenogenetic mayfly, Centroptilum triangulifer. Eleven percent of females sampled contained ciliates. All infected females oviposited ciliates when their abdomen contacted the water surface during oviposition trials. Furthermore, none of these females possessed eggs, yet all females contained gonadal tissue remnants within their abdomens, suggesting that ciliates were directly consuming gonadal tissue.