The lobate ctenophore Mnemiopsis leidyi is a periodically abundant and voracious plankton predator in coastal waters along the east coast of the United States. In the 1980s it was accidentally introduced to the Black Sea where it caused a dramatic reduction in fisheries. We investigated how M. leidyi is affected by infestation with parasitic larvae of the sea anemone Edwardsia lineata. Infested M. leidyi contained 1–30 (median 7) E. lineata larvae. Within M. leidyi most larvae had their mouth in the gastrovascular system near the aboral end of the pharynx. Parasitic E. lineata ingested all food previously ingested and pre-digested by M. leidyi. Non-infested M. leidyi had higher growth rates than infested individuals, which had zero or negative growth rates. Egg production was similar for infested and non-infested M. leidyi of similar size. Simulation based on the empirical data suggests that growing, non-infested, M. leidyi are expected to have a larger life-time egg production than infested shrinking individuals. E. lineata could be at least partially responsible for the sharp decline of M. leidyi populations in fall in US coastal waters. Advantages and disadvantages of E. lineata as a potential candidate for the control of the artificially introduced M. leidyi population in the Black Sea are discussed.