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 occurrence 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.