A sample of 40 fire—scarred trees was used to reconstruct the frequency and size of fires during the past 300—400 years in northern Yellowstone National Park. Best estimates of frequency suggested mean intervals of about 20—25 years between fires, after adjustments had been made for the recent influence of modern man. Agreement in fire dates over wide areas suggested the occurence of 8 or 10 extensive fires in the past 300—400 years. Euro—American man has substantially reduced the natural fire frequency for about 80 years and has thus contributed to changes in plant succession.