We have performed Monte Carlo simulations to study the behavior of a single nanoparticle and a single homopolymer chain in ordered structures of diblock copolymers. Our results show that the microphase-separated copolymers can be used to position nanoparticles or homopolymers according to their interactions with the two blocks. This is consistent with recent experimental observations. We also observe that, while short neutral homopolymers segregate at the A-B interfaces, longer neutral homopolymers exhibit an apparent preference for the cylinders over the matrix, mainly due to geometrical considerations associated with the curvature of A-B interfaces. (C) 2003 American Institute of Physics.