The frequency of forest fires during the past 600 yr was studied on 281 sample plots within boreal forest ecosystems in a river valley in northern Sweden. Fire scars in living and dead trees were used for dating past fires. Before fire suppression started in the 19th century the coniferous forest had burned at a mean interval of 80 yr; in contrast, the mean time since the latest fire was found to be 155 yr. Fire frequency was studied in relation to physiographic and biotic features, e.g. exposition, surface relief, edaphic conditions, and vegetation type. It was found that all these factors have influenced the vegetation pattern induced by past fires, and led to a widespread vegetation mosaic. Fire maintained the diversity and long-term stability of the forest and represented a natural factor in the boreal forest ecosystem. Due to efficient fire suppression during the past two centuries, fire is no longer a rejuvenating factor in the forests.