The synthesis of large core-shell particles suitable for applications as rigid foams is reported by means of a seeded suspension polymerization. Suspension polystyrene particles were used as a seed and methyl methacrylate as second stage monomer. Structured particles with a core of polystyrene and a shell composed by PMMA domains densely dispersed in a polystyrene matrix were obtained. The size and concentration of these domains (shell) decreased from the outer shell to the core of the particle. The particle morphology was largely controlled by the limitations to the diffusion of the MMA through the polymer particle. Therefore, increasing the swelling time resulted in a higher amount of PMMA incorporated and in a higher shell thickness. The effect of the initiator was relatively minor. The molecular weights of shell polymer were substantially higher than those of the PMMA obtained through standard batch and semibatch suspension polymerization.