Nucleation is the limiting stage in the kinetics of polymer crystallization. In many applications of polymer processing, nucleation is enhanced with the addition of nucleating agents. 1,3:2,4-Bis(3,4- dimethylbenzylidene)sorbitol or DMDBS is a nucleating agent tailored for isotactic polypropylene (iPP). The presence of DMDBS changes the phase behavior of the polymer. For high enough temperatures, the system iPP-DMDBS forms a homogeneous solution. However, in the range of concentration spanning from 0 to 1 wt % of DMDBS, the additive can phase separate/crystallize above the crystallization temperature of the polymer, forming a percolated network of fibrils. The surface of these fibrils hosts a large number of sites tailored for the nucleation of iPP. The aim of this paper is to investigate the combined effect of flow and DMDBS phase separation on the morphology of iPP. To this end, we studied the rheology of phase separated iPP-DMDBS systems and its morphology with time-resolved small-angle X-ray scattering (SAXS). The effect of flow is studied combining rheology, SAXS, and a short-term shear protocol. We found that, with phase separation, DMDBS forms fibrils whose radius (∼5 nm) does not depend on the DMDBS concentration. The growth of these fibrils leads to a percolated network with a mesh size depending on DMDBS concentration. Compared to the polymer, the relaxation time of the network is quite long. A shear flow, of 60 s-1 for 3 s, is sufficient to deform the network and to produce a long-lasting alignment of the fibrils. By design, lateral growth of iPP lamellae occurs orthogonally to the fibril axis. Therefore, with crystallization, the preorientation of DMDBS fibrils is transformed into the orientation of the lamellae. This peculiarity is used here to design thermomechanical histories for obtaining highly oriented iPP morphologies after shearing well above the melting point of the polymer (i.e., without any undercooling). In contrast, when shear flow is applied prior to DMDBS crystallization, SAXS showed that iPP crystallization occurs with isotropic morphologies. © 2008 American Chemical Society.