Presents frequencies, intensities, and influences of fire on stand structure and composition on the Bitterroot National Forest in west-central Montana. Three study areas were established, each having a wide range of elevations and forest types. Findings are based upon study of nearly 900 individual fire scars on living trees, and on age-classes of shade-intolerant trees attributable to fire. During the period from 1600 to 1900 fires were frequent in most habitat types, and substantial amounts of forest survived most fires. Some high-intensity stand-destroying fires were also detected in certain habitat types on each study area. Results show that fire was historically a major force in stand development, but that is has been of minor significance during the past 50 years, possibly because of organized fire suppression.