The Barus effect in polypropylene and polystyrene blended with a variety of fillers at various concentrations was investigated using a capillary extrusion rheometer. If the die swell c~ is defined as the square of the ratio of the extrudate diameter d to the die diameter D, it is found to depend on the apparent shear stress r'w. Below a certain value of V' w the relation ~ = B v~ applies. The die swell, c~ M, of a filled polymer depends on the type, size and volume fraction of the filler. In particular, A increases as the volume fraction increases and is largest for powders, smaller for flakes and smallest for fibres, whereas B shows the opposite trend but to a lesser extent.