We describe the development of phase morphology in polypropylene (PP)/ethylene–butene copolymer (EBM) blends in both extrusion through dies and in subsequent melt spinning to form filaments. This immiscible blend has a very low interfacial tension around 0.69 dynes/cm. In the die extrusion process, at low extrusion rates, the dispersed PP phase was sheared into fibrils; at higher extrusion rates, the PP fibrils formed an encapsulation layer near the die wall first, then it broke into droplets because of both interfacial tension and long residence time. These droplets agglomerated together to form a network. In melt spinning, the dispersed phase was also drawn down into fibrils, which coalesced into a continuous layer on the outer surface of the filaments at higher drawdown rates. POLYM. ENG. SCI., 50:1969–1977, 2010. © 2010 Society of Plastics Engineers