The hydrodynamic and thermodynamic behavior of seven narrow-distribution, atactic polyla-methylstyrene) (PaMS) samples in toluene at 25 °C and benzene at 30 °C (PCS only) were studied by using both photon correlation spectroscopy (PCS) and time-averaged light scattering (TALS). In the molecular weight range 5 X 103-106, changes in the slope of log-log plots of the chain self-diffusion coefficient, D0, vs. chain molecular weight reveal that effectively monodisperse, atactic PaMS in a good solvent at room temperature undergoes a gradual transition in chain statistics from Gaussian to excluded-volume behavior. In the excluded- volume region, > 105, D0 scales as the negative 0.59 power of molecular weight, in good agreement with both the Flory prediction of 3/5 and the recent renormalization group calculation of 0.588, while the interpenetration function, is constant at 0.140, comfortably situated in the middle of a number of predictions of its value. These results support the view that in terms of both its coarse thermodynamic and hydrodynamic behavior, the linear chain in a good solvent may be replaced by its respective equivalent hard spheres. The ratio of the values of the PaMS chain hydrodynamic radius to its radius of gyration was measured to be about 0.5, in reasonable agreement with the recently calculated value of 0.537. Finally, the behavior of the PaMS chain-chain hydrodynamic interaction parameter, kD, was found to be best described by the treatment of Yamakawa. For solution concentrations expressed in volume fraction units, kDø was measured to be about 2.7, in reasonable agreement with the Yamakawa prediction of 2.5 ± 0.2. © 1981, American Chemical Society. All rights reserved.