A survey of particle size distributions in underground coal mines was conducted by NIOSH. Data were collected at ten underground coal mines working in seven different coal seams. Samples were collected with newly developed cascade impactors at selected points, including the continuous miner operator's and shuttle car operator's breathing zones, various haulage roads, transfer points such as belt feeders, and intake air streams. The mass frequency distributions were determined. Size distributions for all points except the belt feeder had a primary size mode of about 17 to 20 microns. All areas except the continuous miner and feeder showed a secondary size mode of about 5 to 8 microns. For the feeder, the available data indicated a single size mode with a mean of about 8 to 10 microns. The authors conclude that there is a bimodal character to the dust in coal mines. The primary source of the larger, 17 to 20 micron, mode may come from operation of the continuous feeder. Knowing the particle size distribution in underground coal is important for determining risk of upper airway respiratory diseases in coal miners, and developing control techniques that reduce dust exposure. For example, wetting down haulage roads may eliminate exposures to secondary resuspended dusts. Additionally, size distribution information is essential for determining the accuracy and precision of instruments that are used to assess compliance to dust standards.