A study of extinctions of North American fish during a 100 year period (1889-1989) indicated that 40 taxa, including 27 species, 13 subspecies and 3 genera were lost during this time. Predation and competition by exotic fish factored in 63% of these extinctions, and hybridization with native species is cited in an additional 38% of cases. Management of exotic species is important for the protection of native fishes. Understanding the tolerance limits of exotic species may allow managers to identify areas most susceptible to an exotic invasion. Information on species tolerances could also be used to create or mimic unsuitable conditions for exotic fishes. This could be used in inhibit range expansion or infestation of river reaches by discouraging passage though or persistence in an area, or to create exclusive refugia for important life states of native species that are negatively impacted by exotic fish. We are using standardized data of fish presence and environmental characteristics from approximately 900 randomly selected sites across the 12 western states, including Arizona, California, Colorado, Idaho, Montana, Nevada, North Dakota, Oregon, South Dakota, Utah, Washington, and Wyoming. This data is being collected as part of the U.S. Environmental Protection Agency Western Pilot Study of the Environmental Monitoring and Assessment Plan (E-Map) Project. We are using this data to relate the distribution of 10 exotic fish species from four families (Centrarchidae, Ictaluridae, Cyprinidae, and Poeciliidae) in the western United States to environmental parameters (water quality, habitat characteristics, fish community characteristics). The 10 species examined in this study are those that have demonstrated major impacts on western native fishes. The student completed a MS thesis on this study in late 2003 and is now preparing results for publication.