We compare the linear and nonlinear rheological response of three entangled polystyrene solutions with the same concentration of polymer, but diluted using different solvents. The three solutions have exactly the same physical tube model parameters when normalized to the same time scale. Although the three solutions behave identically in small amplitude oscillatory shear flow, they behave markedly different in large strain extensional flow. The experimental observations presented here directly demonstrate that the tube model and its governing parameters are insufficient to describe the nonlinear rheological behavior of entangled polymer solutions. We introduce a new hypothesis that relates the observed nonlinear behavior of three different polymer solutions to the existence of nematic interactions between polymer–solvent and polymer–polymer molecules.