A theoretical and experimental study was made of the hydrodynamic drag on a small sphere moving relative to a plasma in local thermodynamic equilibrium. The results have application to velocity measurement in ionized gases using the technique of injecting small particles and following their motion as a function of time. The previously available sphere drag results for fluids with properties which vary linearly with temperature are shown to be in error by as much as 40 percent for an argon plasma at 13,000 deg K, for example, because large temperature gradients occur in the vicinity of the sphere where fluid properties vary nonlinearly with temperature. A new drag calculation for a sphere Reynolds number of zero has been made for an argon plasma taking into account nonlinear variations of transport properties with temperature. Sphere deflection measurements in an argon plasma have been made at Reynolds numbers between 0.3 and 1.5. The interpretation of these measurements in terms of sphere drag is subject to confirmation of the plasma transport properties used in the data reduction, but the difference between the measured drag and the drag calculated for zero Reynolds number appears to be approximately the same as in the classical case for invariant fluid properties.