The coefficient of heat transfer to a mobile sphere in a fluidized bed of relatively large particles has been measured. Measurements were also made for U < U-mf, where the solids are stationary. One feature of these experiments is that the heat-transfer sphere was of a size comparable to that of the bed particles. Two methods of measuring heat-transfer coefficients in the bed were employed: (i) The rate of cooling of a sphere, initially hot, was measured by means of a tiny inserted thermocouple with fine lead wires connected to a temperature recorder external to the bed, and (ii) a small number of spherical CO2 particles (dry ice) were put into an air-fluidized bed of inert particles. The subsequent concentration of CO2 in the off-gas from the bed provided a measure of the evaporation rate of the dry ice particles and, hence, of the heat-transfer rate to each dry ice particle. Good agreement between values of h from the two methods implies that the thermocouple lead wires of method W did not restrain the free motion of the heat-transfer sphere. The meaning of the heat-transfer coefficient is discussed with regard to its applicability to a transient experiment, and a criterion is suggested for determining when it is valid to derive a heat-transfer coefficient-a steady-state parameter-from a transient experiment. At high Reynolds numbers (100 < Re-mfs < 830) and with small spheres in a bed of large particles (0.2 < d(s)/d(p) < 2.75), the dominant mechanism of heat transfer is found to be to the flowing gas. In this case, Nu = 2 + 1.0Re(mfs)(0.6) (d(s)/d(p))(0.26).