The rise velocity, V, of a single sphere, released in the bottom of a bed of sand fluidized by air, was measured: the sphere had a diameter of 9.0 or 13.2 mm; its density ranged from 900 to 12 10 kg/m(3). These experiments with a single sphere used: (i) a bubbling bed, diameter 141 mm, with 1.05 < U/U-mf < 2.00, (ii) a slugging bed, diameter 24 mm, with 1.70 < U/U-mf < 3.20. Here U is the fluidizing velocity; U = U-mf at incipient fluidization. It was found that, for each sphere in a given bed, V = V-mf + C(U - U-mf): the constant C was up to 10 times larger for bubbling beds than slugging beds. The rise velocity at incipient fluidization, V-mf, is governed, for both types of bed, by the apparent viscosity of the incipiently fluidized bed. Therefore, Stokes's law was used to predict V-mf, but using an important modification: since each buoyant sphere appears to carry on its top a defluidized 'hood' of particles, Stokes's law was applied to the composite 'particle' consisting of the sphere plus its hood. Analysis of the measured V-mf then gave the volume of the hood, in agreement with direct measurements of it above a fixed cylinder in a two-dimensional bed. In addition, the analysis gave the apparent viscosity of the incipiently fluidized bed to be 0.66 Pa s, in excellent agreement with the estimate of Grace (Can. J. Chem. Eng. 48 (1970) 30) for similar sand. (C) 2004 Elsevier Ltd. All rights reserved.