Concentrations of mercury (Hg) in soil profiles and vegetation were examined in unburned areas and in areas recently burned by wildfires of low, medium, and high fire severities in western Wyoming. Paired unburned and burned sampling sites with similar tree species composition, forest stand age, climate, and geological substrate were studied. Results indicate that Hg release from forest fires is dependent on the tree species composition of the forest, which affects prefire Hg accumulation, as well as the forest fire severity. On the basis of an average of 2.7 ¿ 106 ha of forest and shrubland burned annually in the United States we estimate that wildfires and prescribed burns in the United States release 19 to 64 ¿ 106g of Hg annually. This represents between 13 and 42% of the estimated United States anthropogenic Hg flux of 150 ¿ 106g.