Fluidized bed viscosity was studied experimentally in a series of reduced gravity parabolic flights aboard NASA's KC-135 aircraft. Silica sands of two different size distributions were fluidized by air. The experimental setup incorporated instrumentation to measure the air flow rate, the pressure drop and the apparent viscosity of the fluidized sand at a wide range of the shear rate. The fluidization chamber had transparent walls to allow visualization of the structure changes involved in fluidization in reduced gravity. Experiments were performed over a broad range of gravitational accelerations including microgravity and double gravity conditions. The results of the flight and ground experiments reveal significant differences in overall void fraction and hence in the apparent viscosity of fluidized sand under microgravity as compared to single gravity conditions.