Life-cycle characteristics and habitat processes can potentially interact to determine gene flow and genetic structuring of parasitic species. In this comparative study, we analysed the genetic structure of two freshwater trematode species with different life histories using cytochrome c oxidase I gene (COI) sequences and examined the effect of a unidirectional river current on their genetic diversity at 10 sites along the river. We found moderate genetic structure consistent with an isolation-by-distance pattern among subpopulations of Coitocaecum parvum but not in Stegodexamene anguillae. These contrasting parasite population structures were consistent with the relative dispersal abilities of their most mobile hosts (i.e. their definitive hosts). Genetic diversity decreased, as a likely consequence of unidirectional river flow, with increasing distance upstream in C. parvum, which utilizes a definitive host with only restricted mobility. The absence of such a pattern in S. anguillae suggests that unidirectional river flow affects parasite species differently depending on the dispersal abilities of their most mobile host. In conclusion, genetic structure, genetic diversity loss and drift are stronger in parasites whose most mobile hosts have low dispersal abilities and small home ranges. An additional prediction can be made for parasites under unidirectional drift: those parasites that stay longer in their benthic intermediate host or have more than one benthic intermediate hosts would have relatively high local recruitment and hence increased retention of upstream genetic diversity.