The U.S. Bureau of Mines determined minimum dust layer ignition temperatures on a hot surface for several dusts, using a test procedure recommended by the National Academy of Sciences. The dusts included coal, three oil shales, lycopodium spores, corn starch, grain, and brass powder. For a few of the dusts the effects of particle size and layer thickness on the minimum ignition temperatures were examined. Test results were repeatable and reliable for the fuels, the lycopodium, and the brass powder. The minimum hot-surface ignition temperatures of 12.7-Mm-thick layers of these dusts ranged from 160 deg. C for brass to 290 deg. C for 20- gal ton-1 oil shale. Flaming combustion was observed only with the brass powder. The minimum ignition temperatures decreased with thicker layers and with smaller particle sizes. Some difficulties were encountered with the corn starch and grain dusts. During heating, the starch charred and expanded; the grain dust swelled and distorted. The test was found acceptable for the purpose of determining the minimum layer ignition temperature of a variety of dusts. To prevent fire hazards due to smoldering or flaming dust layers, the temperatures of surfaces on which combustible dusts accumulate should be lower than the minimum hot-surface ignition temperatures of the dusts.