We report a fast preparation method for cobalt (Co) nanowires of about 59 nm in diameter and up to 30 mu m in length by electroless deposition in an aqueous solution at room temperature under an external magnetic field. The reduction process is observed by in situ monitoring of mixed potentials. When the mixed potential decreases below the oxidation-reduction potential of the Co(II)/Co redox pair (E-Co(II)/Co), spherical Co nanoparticles are formed in the solution and then magnetized by the magnetic field. Attractive dipolar interactions are generated along the magnetic field direction, which results in the self-assembly of Co nanoparticles into nanowires. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3568829] All rights reserved.