Parenteral cystacanths of the acanthocephalan Leptorhynchoides thecatus have been reported from 50 species of fishes, and it has been suggested that transmission of paratenic worms to definitive hosts may play an important role in the life cycle of L. thecatus; however, no study has been conducted to assess the infectivity of parenteral cystacanths. Of 100 parenteral cystacanths taken from the mesenteries of green sunfish, 33 were recovered from experimentally infected largemouth bass, demonstrating the capability of parenteral worms to establish and survive in definitive hosts. These results suggest that transmission of parenteral L. thecatus from sunfish to largemouth bass may play an important role in the life cycle of at least some populations of L. thecatus.