The planar extensional flow resistance of a foaming-grade polystyrene, with and without a blowing agent, was characterized using two dies, each one consisting of a high-aspect-ratio straight rectangular channel followed by a hyperbolic convergent rectangular channel. With the hyperbolic geometry, the fluid near the centerline of the convergent channel was subjected to a constant rate of extension. The shear viscosity was found from the pressure drop along the straight channel, and the planar extensional viscosity was determined from the pressure drop in the convergent channel, taking into account the pressure drop due to shearing. Values of the planar extensional viscosity are compared with values of the uniaxial extensional viscosity measured with a shear-free fixture. Tests with a polystyrene and with a solution of 5% carbon dioxide in the polystyrene revealed that the gas caused significant reductions in both the shear and extensional viscosities.