A novel technique for the self-assembly of three-dimensional mesoscopic structures in a forced fluid flow by employing a magnetic field is described. There are advantages of using magnetic fields for this purpose: unlike many other forces, a magnetic force is effective even from a distance, permitting "action at a distance," it is also localized, and competition between the magnetic force and fluid shear enables unique self-assembled ferrofluid structures. Herein, a simulation provides insight into the possibility of using magnetic field to assemble colloidal nanoparticles into aggregates. Subsequently, a demonstration experiment is conducted to characterize the development and decay of such aggregates. The analysis provides a basis for developing effective self-assembly techniques for various engineering applications. (c) 2005 American Institute of Physics.