In-flight measurements of the following parameters have been carried out in the smoke from prescribed fires: carbon dioxide, sulphur dioxide, nitrogen dioxide, ozone, particulate matter, integrated light scattering coefficient and ultraviolet radiation intensity. One fire (in Victoria) was of high intensity; the remainder (in Western Australia) were prescribed fires of low intensity designed to reduce the fuel on the forest floor. 1.The proportion of fuel that became particulate matter was 0.44% for the high intensity burn and 2-4% for the low intensity fuel reduction bums. 2. The particulate matter from the high intensity burn was lower in tar and higher in ash than that from the low intensity fuel reduction fires 3.The concentration of particulate matter, d (g m-3), was related to the integrated light scattering coefficient, bs (m-1, at 550 nm), by the relationship d = 0.25bs. 4. Significant quantities of ozone were produced by the action of sunlight on the smoke, but because ultraviolet radiation was strongly attenuated by the smoke, elevated ozone concentrations were found only in the top few hundred metres of the plume. 5. The maximum concentration of nitrogen dioxide in the smoke at no time exceeded 0.024 ppm above the level measured in clean air outside the smoke plume. 6. No sulphur dioxide concentrations greater than 0.01 ppm were encountered.