The fluid-dynamic analogy of a typical spherical particle in a fluidized bed as the same particle suspended under terminal conditions in a pseudo-fluid (composed of the fluid and all the other suspended particles) is shown to yield predictive estimates for the apparent viscosity of fluidized beds, in good quantitative agreement with reported experimental measurements for particle concentrations of up to similar to 40%. For higher concentrations the predictions fall progressively below measured values, leading to order-of-magnitude underestimates for dense beds at close to minimum fluidization conditions. This latter phenomenon might be ascribable to the dominance, under these conditions, of particle-particle interactions. At high particle concentrations, these would give rise to the observed phenomenon of liquid- and gas-fluidized bed apparent viscosities approaching very similar values, and gas pressure having no influence at all. (c) 2006 Elsevier Ltd. All rights reserved.