Bipalium adventitium Hyman, an exotic terrestrial planarian invading North America, is an aggressive predator on earthworms. The spread and ecological impact of this flatworm will be determined in part by the its interactions with potential predators and prey. In laboratory trials, we tested the ability of B. adventitium to prey upon earthworms of different species and sizes. We also tested the predatory responses of six salamander species and two snake species to the flatworms. Bipalium adventitium attacked and ate members of all earthworm species offered and attacked earthworms over 100 times their mass. However, flatworm predatory success was related to the relative size of the prey. The largest prey eaten in our study was 12.1 times the mass of the flatworm that killed it. When attacking, B. adventitium often used a previously undescribed behavior of capping the anterior end of the earthworm, causing subdued escape behavior. None of the amphibians and reptiles tested as predators treated B. adventitium as a regular prey item. Only a few salamanders (2%) struck and ate a flatworm, with most salamanders and all snakes showing little interest in the planarian. Salamanders that consumed flatworms showed no apparent long-term ill effects.