Species of Ribeiroia (Trematoda: Psilostomidae) are known to cause severe limb malformations and elevated mortality in amphibians. However, little is known regarding the number of species in this genus or its relation to other taxa. Species of Ribeiroia have historically been differentiated by slight differences among their larval stages. To better understand the systematics and biogeography of this genus and their potential relevance to the distribution of malformed amphibians, specimens identified as Ribeiroia were collected across much of the known range, including samples from 5 states in the United States (8 sites) and 2 islands in the Caribbean (Puerto Rico and Guadeloupe). A cercaria from East Africa identified as Cercaria lileta (Fain, 1953), with attributes suggestive of Ribeiroia (possibly R. congolensis), was also examined. The intertranscribed spacer region 2 (ITS-2) of the ribosomal gene complex was sequenced and found to consist of 429 nucleotides (nt) for R. ondatrae (United States) and 427 nt for R. marini (Caribbean), with only 6 base differences noted between the 2 species. The ITS-2 region of C. lileta (429 nt) aligned closely with those of the 2 other Ribeiroia species in a phylogenetic analysis that included related trematode genera. This evidence suggests that a third Ribeiroia species exists in tropical Africa. Variation in ITS-2 within R. ondatrae was nonexistent among the 8 populations from North America. Our study further suggests that Ribeiroia spp. originally parasitized Biomphalaria sp., and that a host switch to a closely related snail, Helisoma sp., may have occurred in the lineage represented by R. ondatrae. However, relationships within the Echinostomatidae are not understood well enough to make any robust conclusions at this time.