Over the last decade, nanocrystal growth by oriented aggregation has been established as an important mechanism by which nanocrystals grow, unique-often symmetry defying-morphologies are produced, and defects are incorporated into initially defect-free nanocrystals. First, we demonstrate particle size control in the preparation of ferrihydrite nanoparticles by forced hydrolysis from homogeneous solution. Second, we demonstrate particle size control by exploiting oriented aggregation in the formation of goethite (alpha-FeOOH) nanorods from the ferrihydrite nanoparticles. Specifically, we show that the size of the goethite nanocrystals formed depends directly on the size of the precursor ferrihydrite nanoparticles. These results are expected to be broadly applicable to crystals, of any composition, produced by oriented aggregation. (c) 2006 Elsevier B.V. All rights reserved.