The relationship between the microphase morphology and stress-strain behavior of a polystyrene-block-polybutadiene-block-poly(epsilon-caprolactone) SEC triblock copolymer has been investigated with polystyrene as the major component. The semicrystalline PCL block does not change significantly the basic morphology, but it affects the form of the microdomains, so that S57B27C16 shows a "polygonal" core shell morphology in a PS matrix. This morphology seems to be the consequence of crystallization in the inner PCL cylinders inducing deformation of the microdomains, which may create interfacial stresses. These stresses can generate small microcrazes upon external loading which may be responsible for the high elongations at break in this material. Moreover, the long-range order of the microdomains, which can be affected through the preparation conditions, determines the stress-strain behavior. Copolymers with 57 and 51 wt % PS and a corresponding "PS matrix" exhibit elongations at break up to similar to 800%.