Neorickettsia (formerly Ehrlichia) risticii, the agent of Potomac horse fever (PHF), has been recently detected in trematode stages found in the secretions of freshwater snails and in aquatic insects. Insectivores, such as bats and birds, may serve as the definitive host of the trematode vector. To determine the definitive helminth vector, five bats (Myotis yumanensis) and three swallows (Hirundo rustica, Tachycineta bicolor) were collected from a PHF endemic location in northern California. Bats and swallows were dissected and their major organs examined for trematodes and for N. risticii DNA using a nested polymerase chain reaction (PCR) assay. Adult digenetic trematodes, Acanthatrium sp. and/or Lecithodendrium sp., were recovered from the gastrointestinal tract of all bats and from one swallow. The intestine of three bats, the spleen of two bats and one swallow as well as the liver of one swallow tested PCR positive for N. risticii. From a total of seven pools of identical digenetic trematodes collected from single hosts, two pools of Acanthatrium sp. and one pool of Lecithodendrium sp. tested PCR positive. The results of this investigation provide preliminary evidence that at least two trematodes in the family Lecithodendriidae are vectors of N. risticii. The data also suggest that bats and swallows not only act as a host for trematodes but also as a possible natural reservoir for N. risticii.