Despite concentrated research efforts devoted to the study of the two-phase fluid mechanics of fast fluidized beds (FFBs), the particle phase stresses in FFBs are not well-characterized. To address this problem, measurements were made of the local time-averaged solid pressure in a FFB riser for a wide range of flow conditions and bed locations. The solid-pressure values, which were several orders of magnitude smaller than the gas pressure, were found to vary primarily with the local time-averaged solid concentration. Using constitutive relations from the kinetic theory of particulate flow, values of local time-averaged solid-phase shear viscosity were calculated from the measured solid pressure and solid fraction. The calculated viscosity values were found to be intermediate within the rather wide range of estimated values previously used in various two-fluid models for dilute vertical gas-particle flow.