The “pygmaeus” microphallids (MPG) are a closely related group of six digenean (Platyhelminthes: Trematoda) Microphallus species that share a derived two-host life cycle in which metacercariae develop inside daughter sporocysts in the intermediate host (intertidal and subtidal gastropods, mostly of the genus Littorina) and are infective to marine birds (ducks, gulls and waders). Here we investigate MPG transmission patterns in coastal ecosystems and their diversification with respect to historical events, host switching and host-parasite co-evolution. Species phylogenies and phylogeographic reconstructions are estimated on the basis of 28S, ITS1 and ITS2 rDNA data and we use a combination of analyses to test the robustness and stability of the results, and the likelihood of alternative biogeographic scenarios. Results demonstrate that speciation within the MPG was not associated with co-speciation with either the first intermediate or final hosts, but rather by host-switching events coincident with glaciation