Previous studies on extensional properties of entangled solutions have mainly dealt with monodisperse polymers. To investigate the effect of polydispersity on rheological properties of entangled polystyrene solutions, a polydisperse blend with an average molecular weight of 2.65 x 106 was made by mixing 18 components of nearly monodisperse polystyrene. The effect of polydispersity is illustrated by comparing the rheological behavior of this blend with that of a solution of monodisperse polymer with the same average molecular weight and at the same concentration. Shear, small-amplitude sinusoidal flow, step strain in shear, and uniaxial extension are used in this study. The multicomponent system has a much broader relaxation spectrum and a slightly smaller zero-shear-rate viscosity than the solution of monodisperse polymer. The shear thinning behavior of the two solutions are very similar except that the polydisperse system shows an earlier onset of shear thinning. In step strain experiments, the polydisperse system requires a larger time for the completion of the Rouse equilibration process while the damping function is not dissimilar. These differences are attributed to the high molecular weight components in the polydisperse system. As anticipated, the differences in extensional flow are far more dramatic, reinforcing the sensitivity of this particular measurement to the molecular weight distribution. The predictions of the single mode ("toy" model) version of the Mead-Larson-Doi model are compared with the data. The parameters in this model are obtained using a separate set of experiments with monodisperse polymers to develop appropriate scaling relationships. The model provides near-quantitative predictions of these results.