The importance of fire as an ecological disturbance in the Northern Rockies is well accepted. Lightning is generally thought to have been the main source of ignition prior to settlement by Europeans. But writings of explorers and pioneers mention deliberate burning by Indians frequently enough to warrant an investigation of its importance. Interviews with descendants of Native Americans and of pioneer settlers in western Montana suggest that Indian burning was widespread, had many purposes, but was generally unsystematic. Fire chronologies based upon scars on old-growth trees indicate that fire intervals within similar forest types were shortest near Indian-use zones. Comparisons of presettlement fire intervals with those calculated from modern lightning-fire records suggest that Indian-caused fires substantially augmented lightning fires over large areas. As dependence on lightning fires alone may not create or perpetuate certain desirable plant communities or stand conditions, prescribed burning may be needed.