The present distribution of salmonid fishes in Wyoming streams was found to be limited to regions where mean July air temperatures did not exceed 22°C. Much of the present salmonid habitat in streams is predicted to be lost if climatic warming occurs. For increases of 1, 2, 3, 4, or 5°C in mean July air temperature, the geographic area of Wyoming containing suitable salmonid habitat would be reduced by 16.2, 29.1, 38.5, 53.3, or 68.0%, respectively. This loss of geographic range would correspond to reductions of 7.5, 13.6, 21.0, 31.4, or 43.3% in the length of streams having suitable salmonid habitat. In the Rocky Mountain region, increases of 1, 2, 3, 4, or 5°C in mean July air temperature would reduce the geographic area containing suitable salmonid habitat by 16.8, 35.6, 49.8, 62.0, or 71.8%, respectively. As warming proceeds, salmonid populations would be forced into increasingly higher elevations and would become fragmented as a suitable habitat for coldwater fish becomes separated from main river channels and restricted to headwater streams. A geographic information system (GIS) proved useful for combining the various databases necessary to assess the potential impact of global warming on salmonid populations.