Particle motion in a rotating fluidized bed was studied by observing the mixing of two layers of particles of different colors. The particles in the two layers were either nearly identical, except for their color, or were of different density and size distribution. All of the particles were in the Geldart-A classification for conventional fluidized beds. After fluidization, but before mixing, the bed exhibited some fluid-like behavior, that is, the inner surface became radially uniform. For particle layers of the same material, mixing occurred after Umfc was reached, with bubbles observed at the mixing velocity. No experimental difference between Umb and Umfc could be discerned; they either coincided or were nearly equal. Bubbles appeared to be responsible for particle motion and mixing. When the denser particles are placed on the distributor, the mixing behavior was similar to that observed for layers of the same material. However, when the less dense particles are placed on the distributor, mixing is dominated by differences in density and occurs before bubbles are visible.