Carbon nanotubes have been extensively used in isotropic polymer media as mechanical reinforcements or as conductive fillers. New phenomena arise when the polymer matrix is made of an ordered block copolymer. Dispersions of nanotubes stabilized by block copolymers in selective solvents can be used to cast composite films in which the nanotubes are segregated in microdomains of the structured polymer. This concept is here investigated for the case of carbon nanotubes in a poly(styrene)-b-poly(butadiene)-b-poly(methylmethacrylate) terpolymer (SBM). It is observed that casting films of SBM from different solvents in the presence or absence of nanotubes can lead to different morphologies with distinct mechanical and electrical properties. In particular it was found that neat SBM cast from a mixture of cyclohexane and acetone exhibits a cylindrical microstructure. This metastable form is mechanically weak and brittle. But the polymer adopts a stable lamellar morphology in the presence of nanotubes. This phase exhibits much better mechanical properties. The capability of nanotubes to alter the morphology of a block copolymer and to lead to large improvements of mechanical properties appears therefore as a new mechanism of mechanical reinforcement beyond the already reported mechanisms of direct reinforcement and network formation. (C) 2013 Elsevier Ltd. All rights reserved.