The primary electroviscous contribution to the viscosity of suspensions of sulfonated polystyrene latex particles was determined at ionic strengths between 10 −3 and 10 −4 M and shear rates of 650, 9,000, and 16,000 sec. −1. The absence of particle expansion at lower ionic strengths was confirmed by light scattering. The ζ potentials of the sulfonated latex particles at different ionic strengths were determined from measurements of electrophoretic mobility. The observed primary electroviscous effect was compared directly with the theoretical predictions of the equations of Smoluchowski and of Booth. In accordance with theory, the observed effect increased with decreasing ionic strength and was virtually independent of the large change in shear rates. But the results also indicated that whereas Smoluchowski's equation yielded too high an electroviscous effect, values predicted by Booth's expressions were somewhat low.