The behavior of vibrating particle beds under reduced air pressures was investigated using flow visualization. Three size ranges of glass beads, 99, 227 and 332 mu m in mean diameter, were used in the experiment. It was found that the smaller the particle size, the greater the effect of air pressure on the behavior of the bed, and the particle circulation rate decreased with increasing magnitude of the reduced gage air pressure in beds of small and intermediate sized glass beads. As a consequence, the difference in the behavior between beds of different particle sizes diminished under reduced pressures.