The effect of interface interaction and processing conditions combined with the viscosity ratio of elastomer domains to the polypropylene (PP) matrix (eta(d)/eta(m), subscript d and m designate the dispersed phase and the matrix, respectively) on fibril formation was investigated. A close to unity viscosity ratio, high interface interaction between the two phases, and a high shear rate in molding were found to result in fibril dispersion. A low-viscosity elastomer tended to form fibrils also. A high extrusion shear rate seems to have no effect on fibrillar formation, but the dispersion size (D-n) decreased from 0.25 to 0.19 mum with an increasing screw speed from 120 to 160 rpm. The Izod impact strength of blends appears to be optimum when a fine and uniform and nonfibril morphology of the elastomer in PP is formed.