A kinetic theory of the homogeneous fluidized bed is developed which provides information on the mixing process as well as on the physical properties of the bed. The physi-cal model of the fluidized bed is discussed. Energy dissipation in the motion of the solid particles and the dynamics of the motion are considered. Finally, the kinetic theory is used to point out a possible mechanism for axial mixing (diffusion) within the fluidizing agent and to derive equations for the axial mixing coefficient. (D.L.C.)