In the present work, we examine the different flow distortions that are prone to occur during the extrusion of microphase-separated block copolymer melts showing hexagonally packed cylindrical domains and spherical domains. The same successive distortions which appear for homopolymers were observed. Special attention is paid to the initiation of surface cracking, which can be severe enough for the extrudate to split at the die exit. Three SEBS in their microphase-separated state were extruded at several temperatures using a capillary rheometer. Pressure drops were recorded as a function of time, and the melt coming out of the die was filmed. Dynamic rheometry experiments in the linear response region at different temperatures in conjunction with DSC revealed the different morphology transitions for the SEBS at hand. These were checked using SAXS. The results show that the Cox-Merx rule fails at frequencies below 1/tau(p), which represents a percolation time of the system. Extrusion of the SEBS forming hexagonally-packed cylinders of PS in a rubbery matrix showed flow split beyond a critical shear stress. In the case of the SEBS forming spherical PS microphases, no flow split was observed. Films of the melt exiting the die enabled the successive defects to be observed. Moreover, due to the slow relaxation times of block copolymers, initiation and propagation of surface cracks at the die exit were recorded and quantified. A secondary system of cracks that occur on the surfaces created by primary cracks transversal to the extrusion direction was also observed. In the case of high enough shear stresses, multiple secondary cracks occur simultaneously, leading to flow split. It was observed that at "low" mean velocities secondary cracks merge in the center of the core and flow split occurs in several branches. In the case of "high" mean velocities, primary cracks propagate faster than secondary cracks and a central, defect-free, polymer rod is observed at the center of the branches, resembling a continuous peeling. To see if an "apparent" infinite molecular weight is responsible for this system of secondary cracks, a highly-entangled linear (M-w/M-e similar to 400) PB was extruded. Secondary cracks were indeed also observed. (c) 2005 Elsevier B.V. All rights reserved.