Momentum equations governing the vertical distribution of particles of different sizes and densities in a homogeneous fluidized bed are formulated. The particles are presumed to be large enough for the interparticle exchange by pulsating momentum and energy to be carried out mostly through direct collisions. Two main assumptions of principal nature are used : (i) the dispersion of concentrational fluctuations is thought of as that composed of contributions due to separate particulate species, and (ii) the random velocity of any particle is supposed to be proportional to the same random function of time with a coefficient being inversely proportional to the square root of the particle mass. The equations for continua modelling particles of different types are presented in an entirely closed form, by analogy with those of the kinetic theory of multicomponent gases. By way of example, fluidization of binary particulate systems is considered, in which case properties of a macroscopically homogeneous state of a binary bed of ideal mixing and a critical velocity that corresponds to particles of one type being replaced by those of the other type in the upper part of the bed in an inhomogeneously mixed state are considered. A general calculation procedure needed to find out the particle distribution in a fluidized bed of given composition is exemplified and discussed in detail.