Total aboveground biomass (fuels), fuel moisture content, weather conditions at the time of burning, fire behaviour, and quantities of biomass consumed by fire were measured in Artemisia tridentata subsp. tridentata dominated ecosystems at the John Day Fossil Beds National Monument, Oregon, USA. Two fire treatments were investigated: an early autumn burn (Sep. 1987, n = 4 plots), and a late spring burn (May 1988, n = 5 plots). Fires in the autumn treatment plots had greater flame length (4.1 m compared with 1.7 m in spring units), rate of spread (1.6 compared with 0.3 m/s), fireline intensity (6400 compared with 880 kW/m), and total heat load (18-120 compared with 9270 kJ/mÂ²). Reaction intensity and heat-per-unit-area were not significantly different. The mass of fuel consumed was significantly greater in the autumn burn units (9.8 compared with 5.2 t/ha). However, percentage consumption was not significantly different; 93% in autumn units compared with 84% in spring units. It was concluded that there are fundamental differences in fire behaviour and fuel consumption between spring and autumn burning. Differences in parameters associated with active flaming were interpreted to be largely a result of differences in fuel moisture, while differences in fuel consumption and total energy were thought to be related to fuel availability.