We compare the high velocity dewetting behavior, at elevated temperatures, of atactic polystyrene (aPS) and isotactic polystyrene (iPS) films, with the zero shear bulk viscosity (ηbulk) of aPS being approximately ten times larger than iPS. As expected, for aPS the apparent viscosity of the films (ηf) derived from high-shear dewetting is less than ηbulk, displaying a shear thinning behavior. Surprisingly, for iPS films, ηf is always larger than ηbulk, even at about 50 °C above the melting point, with ηf/ηbulk following an Arrhenius behavior. The corresponding activation energy of ∼160±10 kJ/mol for iPS films suggests a cooperative motion of segments which are aligned and agglomerated by fast dewetting.