The Bureau of Mines tested a prototype tapered-element oscillating microbalance (teom) dust monitor for measuring respirable coal mine dust. The monitor uses the inertial behavior of a vibrating element to measure the mass of sampled dust. Tests were performed to determine variability, drift, the effect of humidity, placement of collection filter, and element orientation during analysis, and comparability with gravimetric measurements of respirable coal dust. Variability was about 1.6 Ug. Over about 26 h at 46 pct relative humidity, drift was very slightly negative. When the relative humidity was increased to 78 pct, all units showed a total positive change less than 40 ug over about 70 h. Filter placement variation on the tapered element resulting from inconsistent sizing of the filter mounting bases caused significant error (up to 0.2 Mg). In practice, calibration of the system for each new filter will eliminate that error. Orientation of the element during analysis did not introduce significant error, but care is necessary when mounting the canister that houses the element to avoid damping oscillation of the element. Dust tests were performed in an aerosol chamber using coal dust. Measurements made by the teom dust monitor agreed to within 10 pct of simultaneous measurements by gravimetric personal samplers.