Narrow fractions of polystyrene molecules in the form of uncatenated rings (cycles) were synthesized by reacting bifunctional living linear precursors with an appropriate coupling agent at very low concentrations. The cyclic molecules were separated from the simultaneously formed linear polycondensates by fractionate precipitation. The molecular weights of the cycles ranged from 11 100 to 185 000, thus encompassing the critical molecular weight for entanglements in linear polystyrene. The ring-like nature of these fractions has been investigated by a variety of techniques, including the limiting viscosity number in a good and in a θ solvent as well as neutron scattering in deuteriated cyclohexane. These measurements, part of which are reported here in some detail, display a gratifying agreement with the theoretical predictions reported earlier for uncatenated cyclic polymers. The zero-shear melt viscosities of these cyclic fractions and several others prepared by Fetters and Hostetter to extend the molecular weight range to 390 000 were measured over a wide range of temperatures and compared with the viscosities of linear polystyrenes of similar molecular weights. Above the critical molecular weight for entanglement coupling, no major differences were found between the temperature dependence or the molecular weight dependence of the cyclic polymers and those of their linear counterparts. For the same molecular weight, however, the cycles exhibit somewhat lower melt viscosity values than do the linear molecules. The results are critically compared with those reported by Roovers on similar polystyrene cycles and briefly discussed in terms of recent molecular theories based on snake-like motion (reptation) of chains along a curvilinear tube formed by the constraints of the surrounding entangled matrix.