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.