Interfacial instabilities developed during two-layer flows of LDPE melts were investigated theoretically through viscoelastic FEM analysis as well as experimentally on a flat multi-manifold coextrusion die. During continuous reduction of the minor layer, the wave type appears in the film first whereas the zig-zag type is only visible later, at the much thinner minor layer, which is an opposite order compared to film blowing coextrusion. Moreover, extensional viscosity of the minor layer was found to play a significant role from the interfacial instability point of view. (c) 2005 Wiley Periodicals, Inc.