A structural transformation from a spherical microdomain structure to a cylindrical one in a semicrystalline polystyrene-b-polyethylene-propylene-b-polyethylene (PS/PEP/PE) terpolymer having a total molecular weight of 103 kg/mol under a strong external flow field was observed. Quiescently formed samples made by evaporation of the polymer in decalin show the disordered spherical microdomain structure of polystyrene (PS). The roll cast process generated a cylindrical PS microdomain structure well oriented along the flow direction as determined by TEM and SAXS. We explain the observed structural transformation by an analogy to the behavior of surfactant systems under external forces and furthermore demonstrate that the cylindrical structure is an example of a facilitated equilibrium morphology development by an external force. (C) 2000 Elsevier Science Ltd. All rights reserved.