The blend morphologies of injection moldings of high-impact-strength blends of EPDM or mixtures of EPDM and high-density polyethylene (HDPE) in polypropylene have been elucidated by the use of several classical microscopic techniques. It is demonstrated that, when present, HDPE is located within the EPDM phase, where it probably functions predominantly as a viscosity modifier. Within the core, the dispersed phase has an equilibrium globular shape, whose size can be related to impact strength. Elsewhere, the shape of the dispersed phase accurately reflects the shear forces of viscous flow through the mold cavity. In effect, these small domains serve as tracer elements and provide visual evidence and verification for previously postulated flow patterns. Fast, laminar flow through the center of the cavity to the advancing front and then in a curved path to the wall creates shear patterns such that the dispersed phase is in sheetlike or disk-shaped domains at the melt front and in rapidly crystallized regions near the cold mold wall. When knit lines are formed by union of two melt fronts, these flattened domains are oriented perpendicularly to the flow direction, where they prevent adequate matrix union and probably act as stress concentrators which can provide additional loss of knit line strength. Less easily dispersed, higher viscosity EPDM compositions are recommended where knit-line strength is required.