MAMMOTH Mountain, in the western United States, is a large dacitic volcano with a long history of volcanism that began 200 kyr ago(1) and produced phreatic eruptions as recently as 500+/- 200 yr sp (ref. 2). Seismicity, ground deformation and changes in fumarole gas composition suggested an episode of shallow dyke intrusion in 1989-90 (refs 3, 4), Areas of dying forest and incidents of near asphyxia in confined spaces, first reported in 1990, prompted us to search for diffuse flank emissions of magmatic CO2, as have been described at Mount Etna(5) and Vulcano(6). Here we report the results of a soil-gas survey, begun in 1994, that revealed CO2 concentrations of 30-96% in a 30-hectare region of killed trees, from which we estimate a total CO2 flux of greater than or equal to 1,200 tonnes per day, The forest die-off is the most conspicuous surface manifestation of magmatic processes at Mammoth Mountain, which hosts only weak fumarolic vents and no summit activity, Although the onset of tree kill coincided with the episode of shallow dyke intrusion, the magnitude and duration of the CO2 flux indicates that a larger, deeper magma source and/or a large reservoir of high-pressure gas is being tapped.