A new instrument is described for measuring concentrations of submicrometer particulates with diameters ranging from 0.005 to 1.0 Ám and concentrations as low as 750 particles per cm3 at small diameters and 25 particles per cm3 at the larger diameters. Developed by the Bureau of Mines, the instrument is capable of obtaining size distributions of the sampled particulates using an electrical mobility classification which requires voltages of less than 1000 V over the entire size range. The instrument is normally operated by 115 V ac, but it contains an internal rechargeable battery pack for up to 25 h of continuous dc operation. It has been designed primarily as an incipient fire detector, but can be used as a suspended particulate monitor and size analyzer or as a laboratory instrument for studying combustion aerosols. Representative data are presented, indicating potential uses of the instrument.