The incremental plastic deformation of the crystals of block copolymers made of semicrystalline polypropylene endblocks and amorphous ethylene-r-propylene midblocks occurring during step cycle tensile tests has dramatic effects on the stress-strain curves. This can be understood from the evolution of the morphology and of the microstructure of the crystalline blocks revealed by X-ray scattering experiments. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1428-1437, 2010