Fire has been an important factor in the development of the vegetation of the Snake River Plain. Prior to Euro-American influence, fire helped determine the physiognomy and species composition of many communities. The occurrence of fire varied widely depending on the vegetation present, topography, and other factors. This impact can be detected in many historical documents and inferred from species response to fire. On the upper Snake River Plain, fire was frequently reported and was an important factor in vegetation development. On the lower plain, fire appears to have been less common. Low amounts of fine fuel probably limited the extent. During the postsettlement period, the occurrence of fire has increased throughout much of the Snake River Plain. The introduction of annual grasses, particularly cheatgrass (Bromus tectorum L. ), has altered fuel loads and fuel distribution, which in turn has changed fire intensity and extent. Increased human activity has resulted in greater numbers of ignition sources. The decreased fire-free interval (FFD has greatly modified the presettlement role of fire and the distribution of many species.