The monogenean fish ectoparasites, Gyrodactylus spp. have been studied extensively as model organisms, but two key areas, to date, have received little attention host specificity and local adaptation, and these form the central theme of this thesis. Host specificity was examined for two tropical parasite species infecting the guppy Gyrodactylus turnbulli and G. bullatarudis. Contrary to the expectation that G. turnbulli is a strict specialist, this parasite can infect a range of hosts under both laboratory and semi-natural conditions. Furthermore, the congener species, G. bullatarudis, can infect and reproduce on a temperate fish host, the three-spined stickleback. This thesis also identified different transmission strategies of these two species, which are affected by temperature. Whereas G. turnbulli migrates away from a dead host at high burdens, G. bullatarudis stays with a dead host, even though survival times for both species are similar. Arising from these host specificity studies was the discovery that G. lomi, a parasite of chub, could persist as long-term infections on isolated fish, and two new gyrodactylid species, G. zebrae n. sp. and G. danio n. sp., are described from zebrafish. Finally, local adaptation theory was examined for G. gasterostei, which infects the three-spined stickleback throughout England and Wales but not in the Hebridean Islands. This study ascertained that Hebridean host populations were no more susceptible to G. gasterostei than their mainland counterparts. No evidence of local adaptation was found due to overriding temporal effects. However, local differentiation between populations in their susceptibility and resistance was detected, together with sticklebacks having a much longer immunological memory than previously considered.