In this dissertation I evaluated the ecological and evolutionary mechanisms that promote the spread of non-native parasites infecting novel hosts under contemporary and future climate conditions. Thorough assessment of the impact of introduced parasites and an understanding of the potential effects of climate change on parasite distributions and densities will promote effective conservation of native aquatic biodiversity. The spread of an introduced nematode parasite, Camallanus cotti, infecting the native Hawaiian stream fish, Awaous stamineus, across the Hawaiian Islands provided an opportunity to examine how biotic (densities of introduced & native hosts, individual host traits, genetic diversity) and environmental (land-use, water chemistry) factors promote novel host-parasite interactions. In addition to completing archipelago-wide surveys of parasite distributions and densities in native fish hosts, I characterized geographic patterns of genetic variation in C. cotti to assess gene flow, identify likely conduits of introduction and spread of the parasite across the archipelago. Finally, I utilized a natural precipitation gradient across the Hamakua coast on the island of Hawai`i, as a natural analog to conditions predicted by climate change, to assess the relationship between precipitation and infection of A. stamineus by C. cotti. I found the distribution C. cotti has become decoupled from that of the non-native hosts and that the parasite infects native fishes in remote, relatively pristine watersheds. The abundance, intensity, and prevalence of C. cotti infecting A. stamineus are influenced by a suite of factors, but notably parasitism increases with decreasing precipitation. This finding suggest that infection of native Hawaiian fishes by introduced parasites will increase if climate conditions change as expected. Genetic analysis indicates that C. cotti has spread across the archipelago following an initial introduction on O'ahu with subsequent dispersal to Maui and then underwent stepwise dispersal to other islands in the archipelago. Significant genetic structure also was detected across islands, suggesting that dispersal potential is constrained, which in turn suggests that remediation efforts focusing on invasion hotspots or areas of concern could be effective at reducing parasites loads in native fishes. / acase@tulane.edu