The frictional coefficients of three fractions of polystyrene, each of sharp molecular-weight distribution, have been determined both from diffusion and sedimentation velocity experiments in cyclohexane at 35°C and in cyclohexane at 25°C. The coefficients obtained from the two methods agree within experimental error. These results have been correlated with light-scattering measurements to provide more accurate values of the ``universal constant'' ψ, the ratio of the intrinsic frictional coefficient to the root mean square of the radius of gyration. For cyclohexane at 35°C, the θ temperature, ψ was found to have an average value of 12.9, in close agreement with the theoretical value 12.74 for impermeable coil. For cyclohexene at 25°C, however, ψ was found to have an average value of 14.0. This result conflicts with existing theories, which predict that ψ should decrease in better solvents.